Effect of Syngas Composition on the Combustion and Emissions

energies

Article Eﬀect of Syngas Composition on the Combustion and Emissions Characteristics of a Syngas/Diesel RCCI Engine

Navid Kousheshi 1 , Mortaza Yari 1, Amin Paykani 2,* , Ali Saberi Mehr 3 and German F. de la Fuente 4 1 Faculty of Mechanical Engineering, University of Tabriz, Tabriz 5166616471, Iran; [email protected] (N.K.); [email protected] (M.Y.) 2 School of Engineering and Computer Science, University of Hertfordshire, Hatﬁeld AL10 9AB, UK 3 Faculty of Mechanical Engineering, University of Bonab, Bonab 5551761167, Iran; [email protected] 4 Instituto de Ciencia de Materiales de Aragón (Universidad de Zaragoza-CSIC), María de Luna 3, 50018 Zaragoza, Spain; [email protected] * Correspondence: [email protected]; Tel.: +44-(0)170-7281-098

Received: 7 December 2019; Accepted: 23 December 2019; Published: 2 January 2020

Abstract: Reactivity controlled compression ignition (RCCI) strategy uses two different fuels with different reactivities which provides more control over the combustion process and has the potential to dramatically lower combustion temperature and NOX and PM emissions. The objective of the present study is to numerically investigate the impact of syngas composition on the combustion and emissions characteristics of an RCCI engine operating with syngas/diesel at constant energy per cycle. For this purpose, different syngas compositions produced through gasification process have been chosen for comparison with the simulated syngas (mixture of hydrogen and carbon monoxide). The results obtained indicate that using syngas results in more soot, CO and UHC emissions compared with simulated syngas. Even though more NOX reduction can be achieved while operating with syngas, the engine could suffer from poor combustion and misfire at low loads due to the presence of nitrogen in the mixture. In terms of exergy, both syngas mixtures lead to more exergy destruction by the increase of syngas substitution. Nevertheless, the magnitude of exergy destruction for simulated syngas is less than the normal syngas.

Keywords: RCCI engine; syngas; combustion; emissions; exergy

1. Introduction Highly premixed compression ignition strategies have been proposed as emerging engine technologies by many researchers to decrease the heterogeneous nature of the combustion [1–3]. Most of these strategies are lumped into the low-temperature combustion (LTC) category [4–6], where lower combustion temperatures inhibit NOX formation and longer ignition delay times provide adequate time for better mixing and avoid local fuel rich islands, leading to lower soot formation. The LTC strategy includes homogeneous charge compression ignition (HCCI), partially premixed compression ignition (PPC), and reactivity controlled compression ignition (RCCI) concepts [7]. In order to overcome the disadvantages of HCCI and PPC strategies in terms of direct combustion rate control [8,9], RCCI combustion has been introduced. Kokjohn et al. [10] showed a high potential of combustion controllability with RCCI combustion which was achieved by blending two fuels with diﬀerent reactivities inside the cylinder. In RCCI combustion, a fuel with a low reactivity (e.g., gasoline) is blended with air before entering to the combustion chamber and a fuel with higher reactivity (e.g., diesel) is directly injected through injectors. This method allows the combustion phasing to be

Energies 2020, 13, 212; doi:10.3390/en13010212 www.mdpi.com/journal/energies Energies 2020, 13, 212 2 of 19 controlled by the ratio of these different fuels. By using this technique, in-cylinder stratification can control the combustion duration as well. Application of alternative fuels and exploring their effects on the performance and emissions characteristic of RCCI engines have been conducted by many researchers. The use of the gasoline and diesel in light and heavy-duty engines was proposed by Kokjohn et al. [11]. The results indicated high thermal efficiency with low emissions which are simultaneously achieved by reactivity controlled compression ignition strategy. Walker et al. [12] utilized methane instead of gasoline in an RCCI engine, and discovered that higher loads could be achieved with methane/diesel instead of gasoline/diesel. A similar study was performed by Nieman et al. [13] and showed that natural gas surpasses gasoline performance due to its higher octane number. A large reactivity gradient between these fuels could be useful in controlling the maximum PRR. Alcoholic fuels (i.e., methanol, ethanol, etc.) can be also considered as candidates for low reactivity fuel. Dempsey et al. [14] studied a heavy-duty RCCI engine combustion process using the mixture of hydrated ethanol-diesel. The results showed that the fuel’s local reactivity can be properly controlled in this way, and combustion timing and duration can be easily determined. Benajes et al. [15] used different gasoline and ethanol blends in an RCCI engine, and tested the engine performance from low to high loads. They found that a premixed ratio of 85% of ethanol (in volume) will be needed in order to reach a stable combustion. Several studies have been also performed by researchers to investigate the effects of various parameters including fuel ratio, injection strategy, bowl geometry and etc. on the combustion and emissions of RCCI engines [9,16]. Recently, a considerable attention has been given to syngas as a low reactivity fuel. Syngas (an abbreviation for synthesis gas) also called producer gas or wood gas, is mainly a blend of nitrogen (N2), carbon monoxide (CO), hydrogen (H2), methane (CH4), and small fraction of carbon dioxide (CO2). Syngas can be produced by the gasification process of a carbon-containing fuel, such as biomass, natural gas, heavy oil and coal. Syngas can also be produced through fuel reforming with steam and/or air using compact systems, which can be mounted onboard on a vehicle. In-cylinder and external on-board fuel reforming have been identified as a potential way to improve IC engine efficiency and emissions. Sahoo et al. [17] and Bika et al. [18] separately studied the effects of the H2/CO ratio in the syngas on the performance and emissions of a dual fuel diesel engine. Their research implied that the higher amounts of H2 results in higher engine NOX emissions, lower CO and UHC (unburned hydrocarbons) and better brake thermal efficiencies. Moreover, the amount of CO in products depends mainly on the CO amount in the syngas. Rahnama et al. [19] investigated the use of reformer gas produced onboard by a catalytic fuel reformer as an additive in a natural gas/diesel RCCI engine. They concluded that reformate could improve the engine combustion process at low loads since it enhances the burning rate and compensate the low reactivity of natural gas. In another study the effect of adding hydrogen, reformer gas, nitrogen, and a mixture of hydrogen and nitrogen on ignition delay, pressure rise rate, and ringing intensity was investigated by them [20]. The results indicated that combustion was improved at low engine load by using hydrogen and syngas. In addition, it was found that stable combustion and a low pressure rise rate could be achieved at 17 bar IMEP at the expense of higher carbon emissions with nitrogen-rich intake air. Chuahy et al. [21,22] used the RCCI concept to investigate the effects of reformate (containing CO and H2) on the combustion. The results of this study showed that the use of this syngas as the low reactivity fuel is possible over a wide range of H2/CO ratios. They also indicated that larger amounts of H2 in syngas composition would result in more reactive fuel which decreases the duration of combustion and suppresses low-temperature heat release as well. In another study, Chuahy et al. [23] experimentally investigated an ideal syngas composed of 50% hydrogen and 50% carbon monoxide by volume on a RCCI Engine. Results showed the simultaneous reduction of NOX and soot emissions, however, the amount of CO increased by the increase of diesel substitution fraction with syngas. An optimization on important operating parameters such as supply of the fuel, the composition of syngas, and condition of intake on a syngas/diesel RCCI engine was conducted by Jia et al. [24]. The results of this study under a wide load showed that NOX could be kept in low rates, and Energies 2020, 13, 212 3 of 19 reaching the efficient combustion can be simple in high premix ratio regions and early pilot injection of diesel. Results also showed the optimal H2 fraction should be 60–80% and with 75%, the engine has the ability to achieve high thermal efficiency at full load. The effect of hydrogen addition on engine knock in spark ignition engines was assessed by Rutland et al. [25]. Lowering fuel reactivity due to hydrogen addition, in some operating conditions, was one of the results which leads to the increase of the knock resistance. A CFD analysis of syngas-biodiesel combustion on a CI engine was performed by Costa et al. [26]. The results showed that adding syngas (produced from gasification of rubber wood) to the biodiesel, in return for increasing thermal efficiency reduces the combustion efficiency. Like the rest, decreasing NOX formation in exchange for more soot and CO, was reported. According to the literature, it was found that there has been no research in which the actual syngas obtained from various methods of reforming and gasification, is used as the second fuel with low reactivity in RCCI engines. Most of the researchers assume the first two species (H2 and CO) and use this blend as a simulated syngas instead of the actual ones by neglecting the other species. Hence the present study is conducted to numerically evaluate the potential of using various types of syngas mixtures and assess their effects on combustion process and emission characteristics in a syngas-diesel RCCI engine. Three types of syngas gases are selected for comparison with the simulated syngas comprised solely of hydrogen and carbon monoxide. For all cases, a premixed substitution ratio sweep at constant fuel energy is utilized to investigate their effects on emissions and the overall performance. After all, a first and second law analysis are performed to compare the efficiencies and estimate opportunities for further researches.

2. Simulation Modeling and Methods The RCCI engine, as well as models and sub-models used to compute the simulation of the system, are discussed in the following sections.

2.1. Engine Modeling The engine utilized and modeled throughout this work is a Caterpillar 3401 Single Cylinder Oil Test Engine (SCOTE) being experimentally used by Chuahy et al. [23] at the Engine Research Center, Madison. The engine is typical of a heavy-duty size-class diesel engine whose speciﬁcations could be found in Table1.

Table 1. Engine speciﬁcations [23].

Parameter Value Displacement [Lit] 2.44 Stroke [mm] 165.1 Bore [mm] 137.2 Con. Rod Length [mm] 261.6 Number of Valves 4 Compression Ratio 16.1:1 (stock) IVO [◦ ATDC] 335 IVC [ ATDC] 143 ◦ − EVO [◦ ATDC] 130 EVC [ ATDC] 355 ◦ − Swirl Ratio 0.7 Piston Type Articulated Piston Proﬁle Stock Bowl

2.2. CFD Models and Sub-Models The combustion computational modeling employed for the simulation of this study was performed by CONVERGE version 2.3.5 [27]. Table2 shows the sub-models used for this work. Energies 2020, 13, 212 4 of 19

Table 2. Sub-models utilized in simulation.

Phenomena Model and Reference Spray Atomization Kelvin-Helmholtz/Rayleigh-Taylor by Reitz Vaporization Frossling Correlation by Amsden et al. Turbulence RNG k-ε by Han and Reitz Droplet Collision No time counter (NTC) by Schmidt and Rutland Droplet drag Taylor analogy breakup (TAB) by O’Rourke and Amsden Wall ﬁlm formation O’Rourkes Model by O’Rourke and Amsden Combustion SAGE by Senecal et al.

CONVERGE uses Lagrangian droplet approach for the spray sub-model. The spray atomization model used for spray computations follows the KH-RT hybrid approach [28]. The Frossling Correlation model [29] was employed by CONVERGE CFD to calculated the multicomponent vaporization. Turbulence flow was calculated from the re-Normalization Group (RNG) k-ε model introduced by Han et al. [30] which takes into account the effects of compressibility. No Time Counter (NTC) approach proposed by Schmidt et al. [31] been used to model the droplet collision. The dynamic drag model was utilized in droplet drag calculations [32]. Droplet film formation is also modeled by O’Rourke model [33]. NOX emissions were predicted using an extended Zeldovich method [34]. A phenomenological model has been utilized to calculate soot emissions [34] in accordance with the approach of Hiroyasu [35] which calculates the rate of change of soot mass by:

. . . ms = ms f + mso (1)

. . where ms f and mso are soot’s formation and oxidation rates respectively. Soot’s formation is also proposed by the Arrhenius type expression:

. Es f 0.5 RT ms f = As f mC2H2 P e− (2)

where, mC2H2 is the mass of acetylene which is utilized as the inception species for the formation of soot. As f and Es f are empirical tuning parameters and R is the ideal gas constant. P and T are the cell pressure and the cell temperature respectively. The factor of soot pre-exponential and activation energy in the formation rate has been calibrated with experimental data [23]. CONVERGE uses the SAGE detailed chemical kinetics solver [36] for combustion modeling. SAGE model determines the rates of reaction for each elementary reaction which is utilized to update the concentration of species at each time step. The direct-injected fuel’s physical properties and chemical kinetics were speciﬁed by tetradecane (C14H30) as a diesel surrogate and n-heptane respectively. The results were obtained using a multi-fuel mechanism developed by Ren et al. [37] with 178 species and 758 reactions which has the capability to model diesel and syngas combustion with high accuracy because of incorporating the reaction mechanisms of H2/CO/O2 [38]. Moreover, for computational eﬃciency improvement, the multi-zone chemistry solver is used for the present simulations [39].

2.3. Engine Exergitic Analysis All models of the current study have been performed in a close cycle which starts from inlet valve close state (IVC) and ends at exhaust valve open state (EVO). Hence, the balance of exergy for such a control mass system has to be deﬁned as follows:

Ex Exout Ex = ∆Exsystem (3) in − − des where Ex Exout is the net exergy transferred in or out of the system. Ex and ∆Exsystem shows the in − des exergy destruction and change in exergy of the whole system, respectively. Energies 2020, 13, 212 5 of 19

Thermomechanical exergy which is the combination of the thermal and mechanical exergy and also the chemical exergy of the intake and exhaust ﬂuids predicted as follows:

Xk h i Ex = H (T) H (T ) T S (T) S (T ) RT (4) th j − j 0 − 0 j − j 0 − j=1

k X xj Ex = RT ln (5) ch 0 0 j=1 xj where H and S are the exhaust gas enthalpy and entropy, respectively. T and T0 are the temperatures 0 at current and the reference dead state. xj and xj are the mole fractions of the jth component in the intake (or exhaust) gases and the reference dead state, respectively. R and k are the ideal gas constant and the number of species which are contained in the intake (or exhaust). Fuel exergy (Ex f uel) can be calculated through enthalpies and entropies of reactants and products [40–42]. Since complete combustion is assumed the reactants are fuel and O2 and the products consist of CO2 and H2O as follows:   X X  X X    Ex f uel = Hi Hi T0 Si Si (6) − −  −  reactants products reactants products

Likewise, the transferred exergy through incomplete combustion products (Exincomplete) is calculated in the same approach. The exergy of heat loss which is related to the temperature gradients near the boundaries and output work in the engine are determined from the following equations: Z Z ! EVO T = 0 ExQloss 1 dQ (7) IVC − Tg Z EVO dV ExWork = (P P0) (8) IVC − dϕ where Q demonstrates the heat transfer, Tg is the gas temperature in boundaries like walls. P and dV P0 are the pressures of the cylinder and reference dead state respectively. dϕ shows the variation of cylinder volume versus crank angle. According to exergy balance and referring to the Equation (3), exergy destruction in the engine from IVC to EVO can be estimated as:

Ex = Ex Ex Ex Ex + Ex Ex (9) d f uel − Work − Qloss − exhust intake − incomplete As a matter of fact, in real conditions, the process of heat transfer in an engine is complicated due to the fact that there are several boundaries through which the heat transfers out of the cylinder. In the present study, only the exergy of heat loss between the IVC and EVO states is estimated.

3. Computational Model Validation Based on less time, instead of the whole cylinder geometry, calculations were conducted on a sector with periodic boundaries. Since a seven-hole the injector was mounted on engine, a geometry of a sector with 51.42 degrees was created as shown in Figure1. The ﬁxed embedding option with the length of 0.06 m for covering the liquid and parcel fuel areas, around the injector, was activated to make the calculations more precise. Having analyzed the primary results of grid sensitivity to ensure the grid independence, a 49,000-cell grid and a base cell with the size of 2 mm was chosen due to the best tradeoﬀ between computational time and accuracy. To make the results reliable, created models Energies 2020, 13, 212 6 of 19

Energiesof engines 2020, should13, x FOR be PEER validated REVIEW against the results in the literature. The present simulation validated6 of 19 against the experimental data reported by Chuahy et al. [23] at the University of Wisconsin-Madison.

Figure 1. Geometry and grid of the combustion chamber sector. Figure 1. Geometry and grid of the combustion chamber sector. Firstly, conventional diesel operations ranging from 0.36 to 0.58 of global equivalence ratios were Firstly, conventional diesel operations ranging from 0.36 to 0.58 of global equivalence ratios were chosen to validate the model. Secondly, the equivalence ratio of 0.43 had been selected to study chosen to validate the model. Secondly, the equivalence ratio of 0.43 had been selected to study diesel- diesel-syngas RCCI engine. In this stage, substitutions of 20%, 40% and 60% by energy, were conducted syngas RCCI engine. In this stage, substitutions of 20%, 40% and 60% by energy, were conducted to to evaluate the capability of reduced multi-fuel mechanism on predicting syngas combustion which evaluate the capability of reduced multi-fuel mechanism on predicting syngas combustion which were successfully validated. In all cases, a composition of 50% hydrogen and 50% carbon monoxide by were successfully validated. In all cases, a composition of 50% hydrogen and 50% carbon monoxide volume is used as syngas. Amount of directly injected diesel was maintained constant for all CDC by volume is used as syngas. Amount of directly injected diesel was maintained constant for all CDC cases. Table3 shows a summary of the test conditions for simulation results validation. cases. Table 3 shows a summary of the test conditions for simulation results validation. Table 3. Test conditions for engine. Table 3. Test conditions for engine. Parameter CDC Diesel/Syngas Parameter CDC Diesel/Syngas Speed [RPM ] 1300 1300 Speed [RPM] 1300 1300 Start of Injection [◦bTDC ] 10 10 EquivalenceStart of Ratio Injection[ ] [°bTDC] 0.36, 0.43, 0.50,10 0.58 10 - − SubstitutionEquivalence Ratio [% Energy] Ratio [−] 0.36, 0.43, - 0.50, 0.58 20%, - 40%, 60% CompositionSubstitution of Syngas Ratio [% Energy] - - 20%, 50% 40%, H2 60%, 50% CO Fuel EnergyComposition [J/Cycle] of Syngas 5100 - 50% H2, 50% 5100 CO Fuel Energy [J/Cycle] 5100 5100 In Figure2, the cylinder pressure and HRR, in this work and [ 23] are compared. It indicates the cylinderIn Figure pressure 2, the cylinder and HRR pressure over the and mentioned HRR, in this range work of and syngas [23] substitution are compared. quantities It indicates as well. the Thecylinder absolute pressure errors and in HRR the cylinder over thepeak mentioned pressure range are of 0.02, syngas 0.14, substitution 0.11 and 0.20 quantities MPa while as well. errors The in absolutethe relevant errors crank in the angle cylinder of those peak peak pressure points are are 0.02, 3.86, 0.14, 2.66, 0.11 4.09 and and 0.20 6.72 MPa CA while for the errors cases in (a) the to (d)relevant respectively. crank angle of those peak points are 3.86, 2.66, 4.09 and 6.72 CA for the cases (a) to (d) respectively.

Energies 2020, 13, 212 7 of 19 Energies 2020, 13, x FOR PEER REVIEW 7 of 19

12 12 11 11 10 Experimental 10 Experimental 9 This Work 9 This Work 8 8 7 7 6 6 5 5 4 4 3 3 2 2 1 1 Cyl.Pressure [MPa], AHRR/100 [J/deg] Cyl.Pressure [MPa], AHRR/100 [J/deg] 0 0 -1 -1 -50 -30 -10 10 30 50 -50 -30 -10 10 30 50 Crank Angle [deg] Crank Angle [deg] (a) (b)

FigureFigure 2. Validation 2. Validation for enginefor engine operation operation (a) phi (a) =phi0.43 = 0.43 without without syngas; syngas; (b) Syngas (b) Syngas= 20%; = 20%; (c) Syngas (c) Syngas= d 40%;= 40%; ( ) Syngas (d) Syngas= 60% = 60% [23]. [23]. The emission results are also validated with the ones in Ref. [23] as shown in Figure3. Even though The emission results are also validated with the ones in Ref. [23] as shown in Figure 3. Even the magnitude of NO is accompanied with errors more than soot does, this model has the capability though the magnitudeX of NOx is accompanied with errors more than soot does, this model has the to capture the trends in NO emissions. According to these results, the simulation is quite accurately capability to capture the Xtrends in NOx emissions. According to these results, the simulation is quite done and the model may capture the cylinder pressure and HRR of both CDC and diesel-syngas RCCI accurately done and the model may capture the cylinder pressure and HRR of both CDC and diesel- operations exceptionally well over a wide equivalence ratios and syngas substitution ranges. syngas RCCI operations exceptionally well over a wide equivalence ratios and syngas substitution ranges.

Energies 2020, 13, 212 8 of 19 Energies 2020, 13, x FOR PEER REVIEW 8 of 19

1.4 0.35 1.2 0.3 Experimental 1 Experiment 0.25 This Work This Work 0.8 0.2 0.6 0.15 Soot [gr/KgF]

Soot [gr/KgF] 0.4 0.1 0.2 0.05 0 0 0.36 0.43 0.5 0.58 0% 20% 40% 60% Syngas Mass [%] Equivalence Ratio (b) (a)

20 NOx (gr/Kgfuel) Experiment 10 This Work

0 20% 40% 60% Equivalence Ratio

(c)

FigureFigure 3. 3.Validation Validation for for emissions emissions between between experiments experiments [23 [23]] and and simulations simulations (a) ( Soota) Soot in CDC;in CDC; (b )(b Soot) Soot

inin Diesel-Syngas; Diesel-Syngas; (c) ( NOc) NOxX in in Diesel-Syngas. Diesel-Syngas.

4.4. Results Results and and Discussion Discussion It hasIt has been been shown shown that inthat recently in recently patented patented RCCI engines, RCCI engines, the combustion the combustion mode could mode be effi couldciently be controlledefficiently by controlled the reactivity by gradientthe reactivity which gradient is due to wh theich blending is due to of the fuels blending with low of and fuels high with reactivity, low and inhigh the intakereactivity, port in and the into intake the cylinderport and throughinto the thecylinder injector through respectively the inject [43or–45 respectively]. Actual Syngas [43–45]. mixturesActual asSyngas well asmixtures a mixture as ofwell H2 asand a mixture CO seem of to H be2 and a promising CO seem candidate to be a promising for application candidate as the for lowapplication reactivity as fuel the in low RCCI reactivity engines. fuel Thus, in RCCI in this engines. section, Thus, the same in this tests section, similar the to thosesame intests validation similar to partthose have inbeen validation conducted part withhave threebeen diconductedfferent types with of three syngas different mixtures types obtained of syngas through mixtures gasification obtained processesthrough [46gasification,47]. These processes fuels and [46,47]. their compositionsThese fuels and selected their compositions for comparison selected with thefor simulatedcomparison syngaswith whichthe simulated is consist syngas of only wh H2ichand is COconsist are listedof only in TableH2 and4. TheCO engineare listed operating in Table conditions 4. The engine are theoperating same as theconditions validation are case the in Tablesame3 .as For the all validation cases, a premixed case in substitution Table 3. For ratio all sweep cases, at a constant premixed fuelsubstitution energy is utilized ratio sweep to investigate at constant the fuel eff ectsenergy on theis utilized engine. to investigate the effects on the engine.

Table 4. Syngas components (all units are in vol. %) [46,47].

Simulated Syngas Fuel I Fuel II Fuel III H2 50 13.40 5.10 38.10 CO 50 12.50 13.40 28.10 CH4 - 1.00 1.80 8.60

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Table 4. Syngas components (all units are in vol. %) [46,47].

Simulated Syngas Fuel I Fuel II Fuel III

H2 50 13.40 5.10 38.10 CO 50 12.50 13.40 28.10 CH4 - 1.00 1.80 8.60 CO2 - 8.50 22.00 22.20 Energies 2020, 13, Nx FOR2 PEER REVIEW - 66.10 57.70 - 9 of 19 C2H4 - - - 3.00 C2H6 CO2 -- 8.50 0.10 22.00 -22.20 - N2 - 66.10 57.70 - C2H4 - - - 3.00 Previously, it was reportedC2H that6 CDC engines- suﬀ 0.10er from- high soot- and NOX pollutants, which are due to shorter ignition delay and very high temperatures associated with diesel. This is the reason for Previously, it was reported that CDC engines suffer from high soot and NOx pollutants, which investigating addition syngas into the intake port by substituting part of input energy with diﬀerent are due to shorter ignition delay and very high temperatures associated with diesel. This is the reason syngas compositions.for investigating Figure addition4 illustrates syngas into the the cylinder intake port pressure by substituting and HRR part overof input a range energy of with substitution quantitiesdifferent for all fuelsyngas cases. compositions. Figure 4 illustrates the cylinder pressure and HRR over a range of substitution quantities for all fuel cases.

10 7.6 Simulated Syngas 9 7.4 Fuel I 7.2 8 Fuel II 7 Fuel III 7 6.8 6 6.6 5 -5 -1 3 7 11 15 4 1.6 1.4 3 1.2 2 1

Cyl.Pressure [MPa], AHRR/100 [J/deg] 1 0.8 0.6 0 0.4 -1 0.2 -50 -30 -10 10 30 50 -3 1 5 9 13 17 Crank Angle [deg]

(a)

10 7.8 Simulated Syngas 7.6 9 Fuel I 7.4 8 Fuel II 7.2 7 Fuel III 7 6.8 6 6.6 5 -5 0 5 10 15 2 4 1.8 3 1.6 1.4 2 1.2

Cyl.Pressure [MPa], AHRR/100 [J/deg] 1 1 0.8 0 0.6 0.4 -1 -50 -30 -10 10 30 50 0.2 Crank Angle [deg] -5 0 5 10 15

(b)

Figure 4. Cont.

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12 8.2 Simulated Syngas 11 7.8 Feul I 10 Fuel II 7.4 9 Fuel III 7 8 H2 6.6 7 -5 0 5 10 15 6 5 2.8 2.4 4 2 3 1.6 2 Cyl.Pressure [MPa], AHRR/100 [J/deg] 1.2 1 0.8 0 0.4 -1 0 -50 -30 -10 10 30 50 -5 0 5 10 15 Crank Angle [deg]

(c)

Figure 4. CylinderFigure 4. pressureCylinder pressure and HRR and for HRR RCCI for engineRCCI engine fueled fueled with with diesel diesel and and di ffdifferenterent syngas syngas mixtures mixtures with varying syngas substitution ratios in constant energy per cycle for syngas, (a) 20%; (b) with varying40%; syngas (c) 60%. substitution ratios in constant energy per cycle for syngas, (a) 20%; (b) 40%; (c) 60%. The ratios of H2 mass to the mass of other combustible species in syngas are 0.078, 0.065, 0.025 The ratiosand 0.075 of H for2 simulatedmass to syngas, the mass fuel ofI, II other and III, combustible respectively. An species increase inin this syngas ratio results are 0.078, the peak 0.065, 0.025 and 0.075 forpressure simulated increased syngas, at the same fuel substitution I, II and III, ratios respectively. as well. Therefore, An increase the advanced in this CA50 ratio (crank results angle the peak a 50% burn) higher hydrogen content cases can be attributed to both the higher speed of hydrogen pressure increasedflame and atthe the higher same total substitution fuel reactivity. ratios To make as sure well. of this Therefore, idea, an extra the advancedstudy was performed CA50 (crank in angle a 50% burn)20% higher substitution hydrogen ratio at content the same casesenginecan conditions be attributed which its toresults both is theshown higher in Figure speed 4 by ofthe hydrogen flame anddashed the higher blue line. total As fuel can be reactivity. seen, the higher To make hydrog sureen in of fuel, this results idea, in anthe extrafaster depletion study was of the performed in 20% substitutionfuel and the ratio higher at pressure the same rise enginerate. Since conditions nitrogen occupies which the its majority results of issyngas shown volume in Figure in fuel 4 by the I and II, the maximum pressure in these two cases for all scenarios are lower than the other two fuels. dashed blueIn line.the meantime, As can fuel beseen, II has the lowest higher hydrogen hydrogen content in among fuel, results the other in fuels the and faster subsequently depletion has of the fuel and the higherthe smoothest pressure pressure. rise rate. This Since has to nitrogenbe the conseque occupiesnce of thethe higher majority ignition of syngasdelay which volume results in in fuel I and II, the maximuma longer pressure transition infrom these the combustion two cases of for diesel all scenariosto ignition of are syngas. lower Due than to this the matter other fuel two II has fuels. In the meantime,the fuel lowest II has maximum the lowest bulk and hydrogen local temperatures content as among shown in the Figure other 5, since fuels enough and time subsequently is available has the for heat transfer. smoothest pressure.Figure 5 Thisalso indicates has to bethat the as the consequence syngas substitution of the rate higher increases ignition from 20% delay to 40% which and 40% results in a longer transitionto 60%, fromin the thecases combustion of the fuel I and of dieselII, the Temperatures to ignition ofdecrease syngas. by 1.8 Due and to 6.1% this and matter by 1.9 fuel and II has the lowest maximum5.7% respectively. bulk and This local is temperaturesjust unlike the fuel as shownIII in which in Figure the increase5, since of enoughtemperature time has is been available for heat transfer.evaluated up to 4%. These effects can be justified by the presence of nitrogen and lower hydrogen content. Even though by increasing the syngas substitution, hydrogen in simulated syngas and fuel FigureIII5 alsoand both indicates hydrogen that and as thenitrogen syngas in fuels substitution I and II increase, rate increases however the from effect 20% of tonitrogen 40% and is 40% to 60%, in thedominant cases of in the comparison fuel I and with II, hydrogen. the Temperatures It might be noted decrease that, in by cases 1.8 of and fuels 6.1% I and andII, the by engine 1.9 and 5.7% respectively.may This suffer is justfrom unlikeachieving the a fuelstable III state in which at low theloads increase using these of temperature fuels because hasself-ignition been evaluated up to 4%.temperature These effects could can be too be hard justified to reachby due the to their presence low pressure of nitrogen and temperature and lower natures. hydrogen Therefore, content. very low temperature and pressure cannot be always accounted as an advantage. Even though by increasing the syngas substitution, hydrogen in simulated syngas and fuel III and both hydrogen and nitrogen in fuels I and II increase, however the effect of nitrogen is dominant in comparison with hydrogen. It might be noted that, in cases of fuels I and II, the engine may suffer from achieving a stable state at low loads using these fuels because self-ignition temperature could be too hard to reach due to their low pressure and temperature natures. Therefore, very low temperature and pressure cannot be always accounted as an advantage. Energies 2020, 13, 212 11 of 19 Energies 2020, 13, x FOR PEER REVIEW 11 of 19

FigureFigure 5. Temperature 5. Temperature comparison comparison between between fuels fuelswith varying with varying syngas syngas substitution substitution ratios, ratios, (a) Cylinder (a) Cylinder Bulk BulkTemperature; Temperature; (b) Maximum (b) Maximum Local Local Temperature. Temperature.

FigureFigure 6 indicates6 indicates the amount the amount of four of fouremissions emissions for all for scenarios. all scenarios. Specific Specific emissions emissions unit unithas been has been selectedselected to show to show the amount the amount of emissions. of emissions. Compared Compared to CDC, to CDC, reduction reduction of NOx of NOby X78%by 78%and 97% and 97%in in case casescenario scenario of 60% of 60%substitution substitution of syngas, of syngas, for fu forels fuelsI and I II and in IIexchange in exchange for increasing for increasing by 21% by 21%and and 85% 85%for fuel for III fuel and III simulated and simulated syngas syngas prove prove the fact the that fact with that withthe decrease the decrease of hydrogen of hydrogen content content and and increaseincrease of nitrogen of nitrogen at constant at constant fuel energy, fuel energy, at the at expense the expense of other of other emissions, emissions, NOx NO decreases.X decreases. In this In this regard,regard, NOx NO in FuelX in FuelII, adverse II, adverse to the to other the other pollutants pollutants (Co, (Co,UHC UHC and soot) and soot) has the has lowest the lowest amount amount than thanthe other the other fuels. fuels. This Thisconclusion conclusion is compatible is compatible with withthe previous the previous results results as well. as well. Operating Operating the the engineengine with with fuel fuelII results II results in decr in decreasedeased stratification stratification and and by by increasi increasingng the the substitution substitution ratio ratio of of this fuel, fuel, aa steepersteeper reductionreduction inin NONOxX is is detected. detected. ToTo put put it it another another way, way, reduced reduced local local equivalence, equivalence, less less flame flametemperature temperature and and retarded retarded combustion combustion phasing phasing are the are reasons the reasons which prevents which theprevents NOX emissions the NOx to be emissionsformed. to Comparedbe formed. toCompared the simulated to the one, simulated all three one, types all of three actual types syngas of mixturesactual syngas cause mixtures an increase in causesoot an increase emission. in This soot is emission. due to the This existence is due ofto otherthe existence species inof syngasother species like carbon in syngas dioxide like and carbon nitrogen dioxidewhich and decrease nitrogen the which oxygen decrease amount the in premixedoxygen am air.ount This in lack premixed of oxygen air. prevents This lack the of fuels oxygen from the preventscomplete the fuels burning from and the iscomplete the reason burning for lower and volumetric is the reason effi ciencies.for lower In volumetric addition to efficiencies. these reasons, In the additionrole ofto otherthese combustiblereasons, the speciesrole of likeother CH combustible4,C2H4, and species C2H6 inlike soot CH formation4, C2H4, and shouldn’t C2H6 in be soot ignored. formationSince COshouldn’t traps in be crevice ignored. regions Since where CO traps the flames in crevice usually regions do not where reach, the increase flames of usually this emission do not with reach,the increase increase of of this syngas emission substitution with the in allincrease four fuel of syngas cases, was substitution expected. in That’s all four why fuel compared cases, was to CDC, expected.the amount That’s ofwhy carbon compared monoxide to CDC, increases the amount by 11.4, of 21.4, carbon 56 andmonoxide 11.2% inincreases 60% syngas by 11.4, substitution 21.4, 56 for and 11.2%simulated in 60% syngas, syngas fuel substitution I, II and III for respectively. simulated syngas, fuel I, II and III respectively.

Energies 2020, 13, x FOR PEER REVIEW 11 of 19

Figure 5. Temperature comparison between fuels with varying syngas substitution ratios, (a) Cylinder Bulk Temperature; (b) Maximum Local Temperature.

Figure 6 indicates the amount of four emissions for all scenarios. Specific emissions unit has been selected to show the amount of emissions. Compared to CDC, reduction of NOx by 78% and 97% in case scenario of 60% substitution of syngas, for fuels I and II in exchange for increasing by 21% and 85% for fuel III and simulated syngas prove the fact that with the decrease of hydrogen content and increase of nitrogen at constant fuel energy, at the expense of other emissions, NOx decreases. In this regard, NOx in Fuel II, adverse to the other pollutants (Co, UHC and soot) has the lowest amount than the other fuels. This conclusion is compatible with the previous results as well. Operating the engine with fuel II results in decreased stratification and by increasing the substitution ratio of this fuel, a steeper reduction in NOx is detected. To put it another way, reduced local equivalence, less flame temperature and retarded combustion phasing are the reasons which prevents the NOx emissions to be formed. Compared to the simulated one, all three types of actual syngas mixtures cause an increase in soot emission. This is due to the existence of other species in syngas like carbon dioxide and nitrogen which decrease the oxygen amount in premixed air. This lack of oxygen prevents the fuels from the complete burning and is the reason for lower volumetric efficiencies. In addition to these reasons, the role of other combustible species like CH4, C2H4, and C2H6 in soot formation shouldn’t be ignored. Since CO traps in crevice regions where the flames usually do not reach, increase of this emission with the increase of syngas substitution in all four fuel cases, was Energiesexpected.2020 That’s, 13, 212 why compared to CDC, the amount of carbon monoxide increases by 11.4, 21.4,12 of 56 19 and 11.2% in 60% syngas substitution for simulated syngas, fuel I, II and III respectively.

0.35 6 Ideal Syngas Ideal Syngas Fuel I 0.3 Fuel I 5 Fuel II Fuel II 0.25 Fuel III Fuel III 4 0.2 3 0.15 NOx [gr/KWhr]

Soot [gr/KWhr] 2 0.1

0.05 1

0 0 0%CDC 10% 20% 30% 40% 50% 60% 0%CDC 10% 20% 30% 40% 50% 60% Syngas Mass [%] Syngas Mass [%]

Energies 2020, 13, x FOR PEER REVIEW(a) (b) 12 of 19

200 20 Ideal Syngas Ideal Syngas Fuel I Fuel I Fuel II Fuel II 150 15 Fuel III Fuel III

100 10 CO [gr/KWhr] HC [gr/KWhr] 50 5

0 0 0%CDC 10% 20% 30% 40% 50% 60% 0%CDC 10% 20% 30% 40% 50% 60% Syngas Mass [%] Syngas Mass [%]

Figure 6. TrendTrend of emissions for the studied RCCI engine engine fed fed by by various various fuel fuel mixture mixture against against different diﬀerent

syngas substitution ratios (a) Soot; (b) NONOx;X;( (c) CO; (d) UHC.

Figures7 7 and and8 8 show show the the gross gross indicated indicated e efficiencyfficiency (GIE) and combustion efficiency efficiency of each case at various scenarios, respectively. GIE and combustioncombustion eefficiencyfficiency cancan bebe calculatedcalculated as:as:

GIE = 𝑊/𝑄 (10) GIE = Wind/Qin (10) 𝐸 −𝐸. ∑Pn 𝑚𝐿𝐻𝑉 −𝑚EVO𝐿𝐻𝑉 −𝑚 EVO𝐿𝐻𝑉 −𝑚 𝐿𝐻𝑉EVO 𝜂 = E E = = m f iLHVi m LHVUHC m LHVCO m LHV H (11) f uel𝐸− comb.loss i 1 − UHC∑ 𝑚 𝐿𝐻𝑉− CO − H2 2 ηcomb = = Pn (11) E f uel m f iLHVi Notwithstanding the higher heat transfer of simulatedi=1 syngas due to its high temperature feature,Notwithstanding lower volumetric the higherefficiencies heat transferand also of lack simulated of oxygen syngas in case due of to fuels its high I to temperatureIII compared feature, to the simulatedlower volumetric syngas ecausefficiencies the lower and also work, lack lower of oxygen GIE and in caseconsequently of fuels I tolower III compared combustion to theefficiency. simulated In thissyngas regard cause fuel the II lower with work,27.1% lowerreduction GIE in and comp consequentlyared with lowerCDC has combustion the maximum efficiency. reduction In this in regard GIE. Undoubtedly,fuel II with 27.1% the reductionreduced work in compared cannot be with outweigh CDC hased theby increased maximum syngas reduction quantity in GIE. which Undoubtedly, reduced heatthe reduced transfer workamount. cannot Due be to outweighed the higher flame by increased speed and syngas temperature, quantity whichincreasing reduced H2 fraction heat transfer has a congruentamount. Due effect tothe on highercombustion flame speedefficiency. and temperature,These effects increasingforce the flame H2 fraction to ignite has the a congruent fuels near eff theect creviceon combustion regions eduefficiency. probably These to etheffects higher force reacti the flamevity of to the ignite premixed the fuels charge near theand crevice shorter regions quenching due probablydistance of to H the2. Notice higher that reactivity GIE and of thecombustion premixed efficiency charge and decrease shorter with quenching increasing distance syngas ofH portion2. Notice in thethat engine GIE and at combustionconstant energy. efficiency That decreaseis because with of incomplete increasing syngascombustion portion which in the has engine been atpreviously constant elaborated.energy. That is because of incomplete combustion which has been previously elaborated. Table 5 displays CA10, CA50, and CA90 of all cases over the syngas quantities sweep. It should be noted that, for a specific premixed ratio, simulated syngas has an earlier combustion phasing than actual ones. Contours in cut planes in Figure 9 illustrates the local temperatures inside the cylinder at CA10, CA50, and CA90 for all three scenarios. According to these contours, fuel II and fuel III have the lowest and the highest temperatures respectively among the actual syngas mixtures. Due to the high flame temperature and burning rate of H2 which results in efficient and complete combustion, the temperature would be somewhat higher in the simulated syngas case. In all scenarios, more homogenous combustion can be detected in compare with CDC.

EnergiesEnergies 20202020,, 1313,, 212x FOR PEER REVIEW 1313 ofof 1919 Energies 2020, 13, x FOR PEER REVIEW 13 of 19 Table 5 displays CA10, CA50, and CA90 of all cases over the syngas quantities sweep. It should Table5 displays CA10, CA50, and CA90 of all cases over the syngas quantities sweep. It should be notedTable that, 5 displays for a specific CA10, premixed CA50, and ratio, CA90 simula of allted case syngass over has the ansyngas earlier quantities combustion sweep. phasing It should than be noted that, for a specific premixed ratio, simulated syngas has an earlier combustion phasing than beactual noted ones. that, Contours for a specific in cut premixed planes in ratio, Figure simula 9 illustratested syngas the has local an temperaturesearlier combustion inside phasing the cylinder than actual ones. Contours in cut planes in Figure9 illustrates the local temperatures inside the cylinder at actualat CA10, ones. CA50, Contours and CA90 in cut for planes all three in scenarios.Figure 9 il Accordinglustrates the to localthese temperatures contours, fuel inside II and thefuel cylinder III have CA10, CA50, and CA90 for all three scenarios. According to these contours, fuel II and fuel III have atthe CA10, lowest CA50, and theand highest CA90 for temperatures all three scenarios. respective Accordingly among to thethese actual contours, syngas fuel mixtures. II and fuel Due III to have the the lowest and the highest temperatures respectively among the actual syngas mixtures. Due to the thehigh lowest flame and temperature the highest and temperatures burning rate respective of H2 whichly among results the in efficientactual syngas and complete mixtures. combustion, Due to the high flame temperature and burning rate of H2 which results in efficient and complete combustion, highthe temperature flame temperature would andbe somewhat burning rate higher of H 2in which the simulated results in syngasefficient case. and completeIn all scenarios, combustion, more the temperature would be somewhat higher in the simulated syngas case. In all scenarios, more thehomogenous temperature combustion would be can somewhat be detected higher in compare in the simulatedwith CDC. syngas case. In all scenarios, more homogenous combustion can be detected in compare with CDC. homogenous combustion can be detected in compare with CDC. 0.5 0.5 Simulated Syngas Fuel I Fuel II Fuel III 0.45 Simulated Syngas Fuel I Fuel II Fuel III 0.45 0.4 0.4 0.35 0.35 0.3 0.3 0.25

GIE GIE [%] 0.25 0.2 GIE GIE [%] 0.2 0.15 0.15 0.1 0.1 0.05 0.05 0 0 0%CDC 20% 40% 60% 0%CDC 20%Syngas Mass 40%[%] 60%

Syngas Mass [%]

Figure 7. Thermal efficiency for the studied RCCI Engine fed by various fuel mixtures. Figure 7. ThermalThermal efficiency eﬃciency for the studied RCCI Engine fed by various fuel mixtures.

100 100 95 95 90 90 85 85 Simulated Syngas Fuel I 80 Simulated Syngas FuelFule III

Combustion Combustion [%] Efficiency 80 FuleFuel IIIII Combustion Combustion [%] Efficiency 75 Fuel III 75 70 70 0% 20% 40% 60% 80% 0% 20%Syngas Substitution 40% [%] 60% 80%

Syngas Substitution [%]

Figure 8. Combustion eefficiencyﬃciency for the studied RCCI Engine fed by thethe variousvarious fuelfuel mixtures.mixtures. Figure 8. Combustion efficiency for the studied RCCI Engine fed by the various fuel mixtures.

Energies 2020, 13, x FOR PEER REVIEW 14 of 19 Energies 2020, 13, 212 14 of 19 Table 5. CA10, CA50 and CA90 for ideal syngas, fuel I, fuel II and fuel III in the (a) 20%, (b) 40% and (c) 60%. Table 5. CA10, CA50 and CA90 for ideal syngas, fuel I, fuel II and fuel III in the (a) 20%, (b) 40% and (c) 60%. Ideal Syngas Fuel I Fuel II Fuel III CA10Ideal Syngas5.74 Fuel 5.92 I 5.82 Fuel II5.73 Fuel III (a)CA10 CA50 5.7421.10 5.92 21.99 22.52 5.82 21.37 5.73 (a) CA50 21.10 21.99 22.52 21.37 CA90 48.76 51.40 55.35 49.28 CA90 48.76 51.40 55.35 49.28 CA10 5.31 5.34 5.01 4.84 CA10 5.31 5.34 5.01 4.84 (b) CA50 20.96 22.70 23.78 21.48 (b) CA50 20.96 22.70 23.78 21.48 CA90CA90 52.1552.15 59.83 59.83 65.17 65.17 54.06 54.06 CA10 1.53 1.85 4.41 0.84 CA10 1.53 1.85 4.41 0.84 (c) (c)CA50 CA50 15.0115.01 17.79 17.79 18.66 18.66 16.45 16.45 CA90CA90 35.5735.57 50.70 50.70 51.32 51.32 41.83 41.83

(a) Simulated Syngas Fuel I Fuel II Fuel III

CA 10

CA 50

CA 90

(b) Simulated Syngas Fuel I Fuel II Fuel III

CA 10

CA 50

Figure 9. Cont.

Energies 2020, 13, 212 15 of 19 Energies 2020, 13, x FOR PEER REVIEW 15 of 19

CA 90

CA 10

CA 50

CA 90

FigureFigure 9. Cylinder 9. Cylinder temperature temperature cut cut planes planes at CA10, at CA10, CA50, CA50, and and CA90 CA90 for for simulated simulated syngas, syngas, fuel fuel I, fuel I, fuel II andII and fuel fuel III inIII the in the (a) 20%;(a) 20%; (b) 40%;(b) 40%; (c) 60%(c) 60% substitution substituti byon syngas.by syngas.

In order to better investigating the differences among these four cases (fuels), energy and exergy In order to better investigating the differences among these four cases (fuels), energy and exergy analysis obtained from the first and second law principles were conducted. Through this concept analysis obtained from the first and second law principles were conducted. Through this concept exergy destruction as an extra important term which restricts the maximum achievable output power exergy destruction as an extra important term which restricts the maximum achievable output power has been calculated. The energy and exergy distributions of the four fuels at various syngas masses has been calculated. The energy and exergy distributions of the four fuels at various syngas masses are shown in Figure 10. The total energy can be categorized into four parts. Output power or work, are shown in Figure 10. The total energy can be categorized into four parts. Output power or work, transferred heat, energy losses through exhaust gases and incomplete combustion. At the same transferred heat, energy losses through exhaust gases and incomplete combustion. At the same classification viewpoint, total exergy, divided into five categories. Output power or exergy of useful classification viewpoint, total exergy, divided into five categories. Output power or exergy of useful work, exergy destruction, exergy of heat transfer, exergy transfer through exhaust gases and incomplete work, exergy destruction, exergy of heat transfer, exergy transfer through exhaust gases and combustion. The last three terms can be categorized as one called exergy of transfer. incomplete combustion. The last three terms can be categorized as one called exergy of transfer. As can be observed, fuel II has the most energy and exergy losses as for incomplete combustion As can be observed, fuel II has the most energy and exergy losses as for incomplete combustion among all fuels. This can be justified due to lower temperature which prevents burning of parts of the among all fuels. This can be justified due to lower temperature which prevents burning of parts of fuel. The energy and exergy of the transferred heat through walls, cylinder head and piston surface the fuel. The energy and exergy of the transferred heat through walls, cylinder head and piston which are the boundaries of the combustion chamber, are minimum for fuel II compared to the others surface which are the boundaries of the combustion chamber, are minimum for fuel II compared to the others due to the same reason. It is worthy to mention that the low amount of heat transfer and the high amount of exhaust energy losses is due mostly to the closed system analyses assumption.

Energies 2020, 13, 212 16 of 19 dueEnergies to 2020 the,same 13, x FOR reason. PEER REVIEW It is worthy to mention that the low amount of heat transfer and the16 high of 19 amount of exhaust energy losses is due mostly to the closed system analyses assumption. As shown in FigureAs shown 10, the in Figure exhaust 10, losses the ofexhaust simulated losses syngas of simu arelated much syngas lower thanare much of actual lower ones. than This of impliesactual ones. that simulatedThis implies syngas that simulated has the potential syngas ofhas giving the potential much more of giving mechanical much more energy mechanical in shape ofenergy work in instead shape ofof beingwork rejectedinstead byof being the exhaust rejected gases. by the In thisexhaust point gases. of view, In this fuel poin III hast of the view, nearest fuel behavior III has the to nearest syngas andbehavior fuel II to has syngas the less and capability fuel II has of the work less production. capability of work production. ByBy superficiallysuperficially comparing comparing the the two two figures figures (Figure (Figure 10a,b), 10a, it b), is seenit is thatseen all that the all fractions the fractions of exergy of comparedexergy compared to their to corresponding their corresponding percentage percentage in energy in energy categories, categories, are quantitively are quantitively less. This less. is This due inherentlyis due inherently to the di toff erencethe difference between between energy and energy exergy and of exergy a fuel. of The a factfuel. is The the fact exergy is the of oneexergy fuel of is one the intrinsicfuel is the chemical intrinsic energy chemical of it, energy which of is generallyit, which is larger generally than itslarger LHV than [48]. its Above LHV all,[48]. the Above degradation all, the ofdegradation each energy of category each energy during category the process during of the conversion process of leads conversion to lower leads fractions to lower in the fractions second law in the of thermodynamicssecond law of thermodynamics [49]. Despite of [49]. this, Despite the overall of this, variation the overall trends variation according trends to according the first and to the second first lawsand second are compatible. laws are compatible. AsAs mentionedmentioned before,before, amongamong thethe fourfour fuelfuel cases,cases, thethe exergyexergy ofof workwork ofof simulatedsimulated syngassyngas isis thethe highesthighest inin allall scenarios.scenarios. ByBy contrast, contrast, fuel fuel II II has has the the lowest lowest output output work work which which is shownis shown in Figurein Figure 10 as10 well.as well. Furthermore, Furthermore, the the exergy exergy destruction destruction of simulatedof simulated syngas, syngas, fuel fuel I and I and fuel fuel III areIII are less less than than that that of fuelof fuel II, whichII, which means means more more useful useful work work can can be be extracted extracted from from those those three. three. It It is is provenproven thatthat mostmost ofof thethe exergyexergy destructdestruct inin thethe combustioncombustion processprocess andand forfor thisthis reasonreason aa higherhigher combustioncombustion temperaturetemperature willwill helphelp thethe exergyexergy toto bebe moremore destructeddestructed [[50,51].50,51]. Thus,Thus, allall thethe fuelsfuels havehave lessless exergyexergy destructiondestruction inin 6060 percentpercent case.case. It can bebe alsoalso perceivedperceived thatthat thethe didifferencesfferences ofof exergyexergy destructiondestruction betweenbetween thethe fourfour fuelsfuels reducereduce asas thethe portionportion ofof syngassyngas getsgets higher,higher, whilewhile thethe advantagesadvantages ofof RCCIRCCI combustioncombustion overover CDCCDC remain.remain.

FigureFigure 10. 10. Energy (aand) and exergy exergy distributions (b) distributions of the of the four four fuels fuels at at various various syngas masses. masses.

5.5. Conclusions AA numericalnumerical analysis analysis has has been been performed performed to assess to theassess effects the of effects syngas of (produced syngas by(produced gasification) by comparedgasification) to compared simulated to syngas simulated (contains syngas carbon (conta monoxideins carbon monoxide and hydrogen) and hydrogen) on the engine on the exhaust engine emissionsexhaust emissions and performance and performance of a syngas-diesel of a syngas-diesel RCCI engine RCCI while engine the while energy the amountenergy peramount cycle per is constant.cycle is constant. The following The following major conclusions major conclusions may be stated:may be stated: 1.1. Peak pressure, pressure, PRR, PRR, bulk, bulk, and and max max local local temperatures temperatures increase increase significantly significantly with with increasing increasing the ratiosthe ratios of H of2 mass H2 mass to the to mass the mass of other of other combustible combustible species species in syngas. in syngas. Shortened Shortened ignition ignition delay, advanceddelay, advanced CA50, sharper CA50, sharperHRR, more HRR, NO moreX while NO lessX whilesoot, CO less and soot, UHC CO production and UHC are production obtained for this reason. Therefor hydrogen-rich mixture is the best choice for boosting combustion efficiency.

Energies 2020, 13, 212 17 of 19

are obtained for this reason. Therefor hydrogen-rich mixture is the best choice for boosting combustion eﬃciency.

2. Presence of N2 in syngas composition at all syngas substitution ratios force the temperatures to be decreased. Thus, less NOX is achieved by the fuels with nitrogen in their composition (Fuels I and II). 3. In terms of exergy view, Fuel I and III resemble simulated syngas. Less increase in exergy of incomplete combustion due to their combustion efficiencies are obtained by the increase of syngas substitution ratio. 4. It is expected that the engine may suffer from achieving a stable state at low loads using fuel II due to very low H2 percentage in the syngas. 5. The results indicate that compared to the other methods, the syngas produced by the first way of gasification (fuel I), could be a promising candidate as the low reactivity fuel in RCCI engines. NOX emissions decline as the ratio of syngas increases while soot and UHC emissions remained relatively constant. On the other hand, the exergy of exhaust losses is relatively close to the simulated syngas.

Author Contributions: Conceptualization, N.K., and A.P.; software, N.K.; validation, N.K., A.P. and A.S.M. investigation, N.K. and A.S.M.; resources, M.Y. and G.F.d.l.F.; data curation, N.K., A.S.M., and M.Y.; writing—original draft preparation, N.K., M.Y., A.S.M., and A.P.; writing—review and editing, M.Y. and A.P.; supervision, M.Y., A.S.M., and G.F.d.l.F. All authors have read and agreed to the published version of the manuscript. Funding: The research is supported by a grant from University of Bonab (contract number: 9806). Acknowledgments: The authors would like to thank Chuahy from Oak Ridge National Laboratory for providing the data for simulation and research office of University of Hertfordshire for the support. The authors also thank Convergent Science Inc. for providing a free version of their CONVERGE package for academic purposes. Conflicts of Interest: The authors declare no conflict of interest.

Nomenclature

ATDC After Top Dead Center IC Internal Combustion BTDC Before Top Dead Center IDT Ignition Delay Time CA10 Crank Angle Of 10% Mass Fraction Burned IMEP Indicated Mean Eﬀective Pressure CA50 Crank Angle Of 50% Mass Fraction Burned IVC Inlet Valve Close CA90 Crank Angle Of 90% Mass Fraction Burned IVO Inlet Valve Open CDC Conventional Diesel Combustion LHV Lower Heating Value CFD Computational Fluid Dynamics LTC Low Temperature Combustion CI Compression Ignition PCCI Premixed Charge Compression Ignition EVC Exhaust Valve Close PRR Pressure Rise Rate EVO Exhaust Valve Open RCCI Reactivity Controlled Compression Ignition GIE Gross Indicated Eﬃciency RI Ringing Intensity HCCI Homogeneous Charge Compression Ignition UHC Unburned Hydrocarbon HRR Heat Release Rate

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