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Numerical study of the influence of the photochemical activation of oxygen molecules on HCCI performance Alexander M. Starik, Vyacheslav E. Kozlov, and Nataliya S. Titova Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b00305 • Publication Date (Web): 30 Jun 2017 Downloaded from http://pubs.acs.org on July 2, 2017

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Numerical study of the influence of the photochemical activation of oxygen molecules on HCCI performance A.M. Starik*, V.E. Kozlov, N.S. Titova Central Institute of Aviation Motors, Scientific Educational Centre of Physicochemical Kinetics and Combustion, Moscow, 111116 Russia *Corresponding author. E-mail: [email protected] Keywords: HCCI, excited molecules, ignition, combustion enhancement, NO and CO emission Abstract Numerical analysis of the possibility of the combustion enhancement in HCCI engine operating on methane via the production of O2(a1∆g) molecules or O atoms by resonance laser radiation in the 5 mm thickness layer, located near the cylinder head, is conducted both for the nominal and low load regimes. It is shown that the abundance both singlet delta oxygen and atomic oxygen in such layer accelerates substantially ignition phasing, enhances the combustion and reduces the pollutant emission. Moreover, the advance in the specific work in these cases is higher than the energy put into the gas for producing O2(a1∆g) molecules or O atoms. In doing so, the production of O2(a1∆g) molecules is more effective for the combustion enhancement in HCCI engine. At low load regime, the production of O2(a1∆g) molecules makes it possible to extend the range of stable combustion, to increase notably the engine power (by a factor of 1.4) and to reduce by few times the emissions of CO, unburnt hydrocarbons, and organic compounds. 1 ACS Paragon Plus Environment

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Introduction Nowadays, the internal combustion (IC) engine with homogeneous charge compression ignition (HCCI) is considered as one of the most promising devices that could provide reducing pollutant emission without the usage of catalysts. As is known, HCCI combustion is based on the compression of well mixed fuel-lean mixture to suitable pressure and temperature ensuring the stable ignition. Despite the significant advantages of HCCI engines against the conventional IC engines with spark ignition or diesel engines, they have some substantial disadvantages. Such engines produce significant amounts of CO and unburnt hydrocarbons (UHC) at low load regime.1,2 Another problem, which must be resolved, is the development of approaches aimed at the control of ignition timing and combustion phasing. Nowadays, two concepts for controlling the ignition timing during HCCI combustion are regularly utilized. One of them is the variation of the composition of intake charge or its temperature, and the other concept concerns the injection of some fraction of exhaust gas into the intake port.2-6 However, these approaches have some limitations. For example, direct heating of intake air or the charge requires additional energy and special equipment. The use of exhaust gas recirculation (EGR) leads to retaining the ignition because the combustion exhaust comprises some passive species such as CO2, H2O, N2, and CO, which do not participate in the chain process in the fuel-air mixture. In order to provide the effective control of ignition and combustion phasing, it has been supposed to apply the approaches based on the subjection of the charge before its intake to the cylinder by the electric discharge producing thermally non-equilibrium plasma.7,8 The other way to accelerate the ignition in HCCI engine, which was widely investigated for past few years, is the admixture of small amounts of chemically active species to the charge.9-17 In particular, it has been shown that the oxidation of fuel-air mixture occurs faster under ozone-seeded conditions.9-12 The same effect has been observed when a small amount of NO or NO2 is added in the intake charge.12,15-17 However, nitrogen oxide and nitrogen dioxide appeared to be less effective as additives compared to ozone.12 Conducted analysis13 has exhibited that the production of some amount of oxygen molecules excited to the singlet 2 ACS Paragon Plus Environment

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delta state O2(a1∆g) (SDO) can affect the ignition timing and combustion performance in HCCI engine operating on methane. Moreover, this allows a significant reduction in the output concentrations of NO and CO. In this case, HCCI combustion can be organized at the diminished initial temperature of the charge that makes it possible to increase the thermodynamic work of the HCCI cycle. Somewhat later, it has been demonstrated that the usage of SDO as an additive is more effective than that of atomic oxygen.14 The computations has also shown that the presence of SDO in the charge even in a small amount (~1% of total oxygen) can ensure the ignition of the extremely lean methane-air mixture and arrange the stable HCCI combustion at low load regime with reduced CO emission. It has been demonstrated earlier13 that the maximal increase of output energy is achieved when the excitation of O2 molecules to the a1∆g state occurs at the certain optimal magnitude of crank angle βp. However, the analysis of Starik et al.13 has been performed with the usage of zero-dimensional (0D) single zone model, which assumes that O2(a1∆g) molecules are produced in the whole volume of the fuel-air mixture. In reality, it is more reasonable to produce O2(a1∆g) molecules in a thin layer of combustible mixture in the region over the piston, for example, near the cylinder head at the crank angle equal to βp. In this case, the simple 0D single zone thermochemical model cannot be used, and analysis should be done on the basis of 2D CFD calculations. Besides the activation of O2 molecules via their excitation to the a1∆g state, the other way of the production of highly reactive species in a thin layer can be considered. This concerns the formation of atomic oxygen via photo-dissociation of oxygen molecules by resonance laser radiation at 193.3 nm wavelength.18,19 The goal of the present work is the comprehensive analysis of the possibility of the improvement of combustion performance and reducing the pollutant formation in HCCI engine upon the photochemical activation of O2 molecules in a thin layer of combustible mixture in the cylinder during the compression stroke.

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Methodology Analysis of the influence of the production of SDO molecules or atomic oxygen in a thin layer placed over a piston on the HCCI performance and its emission characteristics was conducted by using 2D axisymmetric CFD model based on Favre averaged non-stationary Navier-Stokes equations for reacting mixture including excited O2(a1∆g) and O2(b1Σg+) molecules. The thermodynamic equilibrium between translational, rotational and vibrational degrees of freedom for all molecules involved in chemical reactions was assumed. CFD runs were performed with the usage of the FLUENT program from the ANSYS-CFD software package.20 Parameters of HCCI engine were chosen the same as they were reported in the guideline of ANSYS program package and presented in Table 1. EGR in the cylinder was assumed to be absent. The calculations ran from the crank angle β= -142°, corresponding to Inlet Valve Closing (IVC), to crank angle β=115° relating to Exhaust Valve Opening (EVO). It was supposed that the walls of the cylinder and the top surface of the piston were cooled, and the temperature at the cooling surfaces was equal to 510 K.21 The quasi-laminar combustion model and realizable k-ε turbulence model were chosen for the simulation. For simplicity, it was assumed that all mixture parameters and composition of the charge were uniform at IVC. The effect of the excitation of O2 molecules to the singlet delta state a1∆g or the generation of O atoms was analyzed assuming that different mole fractions of active species can be produced in a thin layer with the thickness of 5 mm placed near the cylinder head. O2(a1∆g) molecules can be generated due to laser-induced excitation of ground state O2(X3Σg-) to the singlet sigma state O2(b1Σg+) by the radiation with 762.3 nm wavelength and due to subsequent rapid quenching of O2(b1Σg+) to the singlet delta state. It is worth noting that the radiation with this wavelength is generated by diode, dye and solid laser with Al2O3Ti2+ crystal with optical pumping.22 It was also shown previously23 that O2(a1∆g) molecule could be produced through the exposure of air by the radiation of Nd-YAG laser with 1.065 µm wavelength. Oxygen atoms can be produced by the radiation with 193.3 nm generated by 4 ACS Paragon Plus Environment

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excimer ArF laser.18 In order to expose a thin layer of the mixture to laser radiation, it needs to fabricate the cylinder with the small optical window for the introduction of the radiation into the cylinder. The specified amount of O2(a1∆g) molecules can be obtained by multiplying scanning of the irradiated region in the cylinder by a laser beam of radius ~0.1 cm across the cylinder with moderate value of radiation intensive I0~1 kW/cm2. The concentration of atomic oxygen in the irradiated region was determined from the condition that the specific energy, put into the gas, is equal to that spent on the production of given amount of O2(a1∆g) molecules. Note that due to the fact that the energy, required for the production of O atom, is greater than that needed for the excitation of O2 molecule to the a1∆g state, the concentration of O atoms in the irradiated region is smaller compared to the concentration of O2(a1∆g) at identical input energy of laser radiation. Shown in Fig. 1 is the schematic of the HCCI cylinder with the laser system for the exposure of the mixture to laser photons. For the description of ignition and combustion in HCCI engine, the reaction mechanism developed previously for the CH4-O2-N2 mixture and comprising the processes with electronically excited O2(a1∆g) and O2(b1Σg+) molecules13,14 was applied. Note that this mechanism takes into account the recent data of ab initio studies of reaction channels in the H+O2(a1∆g) and CH3+O2(a1∆g) systems24,25 and reproduces properly the known experimental data on the detection of the reduction of ignition delay in the H2-O2 mixture upon the excitation of O2 molecules to the singlet delta state in the low pressure glow discharge.14 The validation of the reaction mechanism is presented in Supplementary materials. All the more, as was recently shown26, this mechanism allowed the authors of that work to describe the experimental data on shortening the ignition delay length in a heated flow reactor filled by CH4-O2 mixture upon the injection of O2(a1∆g) molecules at the entrance of reactor, produced via chemical reaction Cl2+2KOH+H2O2=>O2(a1∆g)+2KCl+2H2O. 5 ACS Paragon Plus Environment

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Results and discussion Consider, at first, how the production of SDO in a thin layer near the cylinder head influences on ignition phasing in HCCI engine. Figure 2 shows the temperature and pressure traces in the basic case (see data in Table 1), in the cases of production of 0.5 and 1% of SDO in total oxygen (herein and hereafter the mole fraction of SDO in total oxygen is indicated) and upon the production of equivalent amount of atomic oxygen in the layer of 5 mm thickness at the crank angle of βp=-32°. It was identified earlier on the base of 0D simulation13,14 that such crank angle is optimal for ensuring the maximal enhancement of ignition in HCCI under SDO production. One can see from Figure 2 that the production of both SDO molecules and O atoms results in the enhancement of ignition. The greater is the concentration of active species (this corresponds to the greater energy of laser radiation put into the gas), the earlier is the ignition event in HCCI cylinder. The presence of SDO molecules in the thin layer near the cylinder head has the greater effect on ignition phasing than that of the equivalent amount of O atoms. The same tendencies were earlier predicted on the base of the single zone thermochemical model for the case when SDO molecules or O atoms were produced in the whole cylinder volume.14 Unfortunately, now there is no experimental data on the influence of SDO molecules or O atoms on combustion phasing in HCCI engine. For the HCCI engine operating on the natural gas/air mixture, Nishida and Tachibana9 observed experimentally the earlier ignition in the case of partial ozonization of the intake gas and proved that ignition timing can be controlled by changing the ozone concentration. They also concluded on the basis of numerical simulation that the influence of ozone addition on ignition angle is about the same as that of direct injection of O atoms. In the basic case, the maximal mass-average temperature (Tev)max is realized at the crank angle βm=10°. Note that the maximal gradient of mass-average temperature, which can be associated with the ignition event, is observed at the smaller value of crank angle β=2.875°. Upon the production of even


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0.5% SDO in total oxygen, the value (Tev)max is reached at the crank angle βm=0.75°, i.e. very close to the top dead center (TDC), β=0°. At 1% SDO content, the value (Tev)max as well as the maximal volume-average pressure (Pev)max is realized still earlier, at βm= -1.125°. As the (Pev)max value is implemented before TDC, the essential energy must be spent at the compression stroke in order to overcome very high pressure resulting from the combustion of the mixture. This must result in the decrease of the thermodynamic work during the whole HCCI cycle, despite the fact that both (Tev)max and (Pev)max values rise. Such conclusion can be also derived from the P-V diagrams depicted in Figure 3a. One can see that these diagrams are slightly different for the cases considered. The thermodynamic work in HCCI cycle is determined by the area of the figure bounded by the line P(V) and calculated by V1

the formula E c = ∫ PdV , where V0 and V1 are the values of volume available for combustion in the V0

cylinder at the crank angles β= -142° and 115°, respectively. In the case of 1% SDO content, the area of P(V) figure is smaller than that in the basic case. Acceleration of ignition due to the production of active species (SDO or atomic oxygen) makes it possible to decrease the initial temperature of the charge in order to ensure the (Tev)max value at the crank angle identical to that in the basic case (βm~10°). The calculations show that upon the production of 1% SDO in total oxygen, the initial charge temperature, at which (Tev)max occurrs at βm~10°, is equal to 431 K, whereas, upon the abundance of equivalent amount of O atoms (0.38%), this temperature should be notably higher (T0=452 K). The temperature and pressure traces for these cases are also depicted in Figure 2. As is seen, the maximal value of mass-average temperature decreases, but the maximal magnitude of the volume-average pressure becomes somewhat higher in these cases compared to corresponding values for the basic case. The behavior of temperature and pressure during the compression-expansion stroke also changes compared to that for the basic case. So, the abundance of SDO or atomic oxygen in the thin layer in the region near the cylinder head provides the greater


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pressure both before the ignition event in the vicinity of TDC and at β>βm~10°. In doing so, the production of SDO molecules is notably more effective in this point of view. The other important issue is that, upon the abundance of SDO in the mixture, the maximal value of the pressure gradient (dPev/dt) is notably smaller than that in the basic case (by a factor of 2.7 and 1.6 for the cases of SDO and atomic oxygen production, respectively). This decreases the load on the walls of the cylinder and on the piston during the compression stroke and provides the longer engine life. These features in the temperature and pressure traces, upon the generation of active species, lead to the greater value of the thermodynamic work. This is seen from the corresponding P-V diagrams shown in Figure 3b. Table 2 summarizes the main characteristics of HCCI combustion including emissions of NOx and CO calculated for the cases considered. When analyzing the data on energy characteristics of HCCI engine in the basic case and upon the production of SDO and atomic oxygen in a thin layer located near the cylinder head, reported in Table 2, one can conclude that even small radiation energy, put into the gas, allows one to increase the specific work during HCCI combustion by 1.8% in the case of SDO production and by ~1% upon the generation of O atoms and, simultaneously, to provide the softer regime of HCCI combustion. Consider now the variation of temperature fields in the cylinder of HCCI engine in the basic case and in the cases when SDO molecules or O atoms are produced in the thin layer near the cylinder head. These fields for different values of crank angle are depicted in Figures 4-6. One can see that, in the basic case (T0=471 K), the ignition occurs at βign~2.5° in the zone with maximal temperature located in the central region (see Figure 4). This region spreads to the mixture layers with smaller temperature adjacent to the zone with the high temperature. As a result at β=6°, the mixture is burnt practically completely in the entire volume over the piston. The estimated “visible speed” of the propagation of the boundary of the high temperature region with temperature 1600 K is equal to


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~50 m/s in cylinder axis direction and ~150 m/s in the radial direction. Because the flame speed in CH4/air mixture is substantially smaller at parameters achieved in the cylinder before the ignition (at TDC T~1180 K and P~40 atm), one can conclude that the combustion inside the cylinder is determined by the subsequent ignition (with longer delay) of the regions with some smaller temperatures, but not by the flame propagation through the mixture. This conclusion is additionally confirmed by the different values of “visible speeds” of the high temperature boundary movement in axis and radial directions. When SDO molecules are produced in the thin layer near the cylinder head at T0=431 K, the ignition occurs in the small region adjacent directly to the cylinder head and located closer to the sidewall of the piston cup (see Figure 5). The charge ignites at βign∼1.5°, i.e. earlier than in the basic case. Thereupon the hot region spreads to the mixture layers with smaller temperatures. In this case, the “visible speeds” of the propagation of the boundary of the hot region are equal to ~6 and 30 m/s in axis and radial directions, respectively, that is appreciably smaller than in the basic case. The decrease in “visible speed” of the hot region boundary is explained by the smaller value of temperature realized at TDC (by ~30 K) due to the smaller initial temperature of the charge, compared to the basic case, and, as consequence, by the longer ignition delay of cooler layers. As a result, at β=6°, the temperature field over the piston is less uniform than in the basic case. Analogous tendencies are observed in the case of O atom production in the layer adjacent to the cylinder head (see Figure 6). In this case, the ignition occurs somewhat later at βign=2.5°, and the “visible speed” of the propagation of the boundary of the hot region is greater (its value is equal to ~16 and ~50 m/s in axis and radial directions) than that in the case of SDO production. Up to the time instant, corresponding to the β value of 6°, the mixture is burnt more completely. In order to clarify the reason of the acceleration of ignition and combustion due to the generation of SDO molecules or O atoms in the mixture layer adjacent to the cylinder head at 9 ACS Paragon Plus Environment

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β= -32°, let us analyze the time evolution of mass-average mass fractions of principal species responsible for the chain mechanism development. Such plots are shown in Figure 7. One can see that the abundance of SDO or atomic oxygen, produced at βp= -32° in the layer placed near the cylinder head, results in the sharp increase of the mass fractions of highly reactive species O, OH and CH3, carriers of chain mechanism in the CH4-air mixture. As a result, the mass fractions of stable species such as H2O and CO2 begin to grow earlier than those in the basic case. In the case of O atom production, the formed atoms eliminate partly just after their formation, and the concentrations of stable species do not change notably at this time interval. It should be emphasized that the recombination of O atoms leads to the formation of rather stable O2(a1∆g) molecules: O+O+M=O2(a1∆g)+M with the higher concentration than those of O, H and OH. The further decrease of SDO molecule concentration is due to the chemical reactions with participation of SDO: CH4+O2(a1∆g)=CH3+HO2,

CH3+O2(a1∆g)=CH2O+OH,

CH3+O2(a1∆g)=CH3O2

and

CH3+O2(a1∆g)=CH3O+O as well as due to its collisional quenching. In the case of direct production of SDO, its concentration is by a factor of 20 greater than that upon production of O atoms. The concentrations of active radicals increase more regularly (without sharp peaks at β∼-32°) and they are notably higher than those in the case of O atom generation. However, at the TDC, the mass-average mass fractions of O atoms in the cases of SDO or O atom formation become smaller (by 20%) than that in the basic case (see Figure 7). The same concerns the concentrations of H, OH, and CH3. Among active species, only SDO molecules have some higher concentration at TDC in the case of SDO production. Another species, which concentration is higher at TDC in the case of SDO molecules or O atoms production (by 50% and 25%, respectively), is H2O2. This molecule, which is rather stable at low temperature, dissociates intensively at T>1000 K (such temperature is realized at the end of compression stroke, near TDC) with the formation of active OH radicals that results in the hot ignition of hydrocarbon fuels.27 The importance of H2O2 decomposition


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for the ignition in HCCI engine was reported earlier.9,28 It was numerically demonstrated28 that the higher H2O2 concentration in HCCI ensures earlier ignition. On the other hand, if to examine the fields of O atom mass fraction, one can notice the difference in its distribution in the cylinder for cases considered (see Figure 8). In the basic case, O atoms spread at TDC rather evenly in the central region of working volume. In the cases of SDO or O atom production, there is a domain where the O atom mass fraction is half as much in comparison with the basic case. This domain adjoins to the layer where active species form and shifts closer to the sidewall due to the weak vortex flow. The ignition stars just in this domain. One can see that the location of this domain coincides with the position of the region with maximal temperature (compare Figure 8 with Figures 5 and 6). Precisely the higher local concentration of active species provides the mixture ignition at some lower initial temperature, when SDO or O atom are produced, and, as consequence, the achievement of maximal temperature value at the same crank angle (βm~10°) as it is in the basic case. The production of SDO molecules or O atoms in the layer adjacent to the cylinder head allows one to operate with the smaller temperature in the HCCI cycle ((Tev)max=2024, 1944 and 1993 K in the basic case and in the cases of production of SDO molecules at 431 K or O atoms at 452 K, respectively). Because the principal mechanism responsible for the NO formation at T>1800 K is the thermal (extended Zeldovich) mechanism29, the concentration of NO in the engine exhaust in these cases becomes lower than that in the basic case, This fact is illustrated by the temperature and NO mass fraction fields at EVO shown in Figures 9 and 10 for the cases considered. It is seen that the maximal concentration of NO is observed in the region located near the axis of the cylinder, where the temperature achieves the major value. The smallest concentration of NO at EVO is provided in the case of SDO production (see also Table 2). The decrease in the NO emission index (EINO), compared to the basic case, is as large as a factor of 3. The emission index for species M is determined as an emitted amount of species M in grams per 1 kg of the fuel; the emission index of NOx is calculated by the 11 ACS Paragon Plus Environment

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formula EINOx=EINO×µNO2/µNO+EINO2, where µNO and µNO2 are the molar masses of NO and NO2. The effect of O atom production on the NO emission is notably smaller. It should be emphasized that the emissions of other N-containing species also decrease in these cases. This is clearly seen from the plots depicted in Figure 11. However, the smaller temperature in the cylinder in the cases of SDO or O atom production leads to the greater emission of CO compared to the basic case. It was shown earlier30,31 that oxidation of CO to CO2 occurs at temperatures of at least 1500 K because CO oxidation is dominated by the reaction CO+OH=CO2+H and is very sensitive to the concentration of OH that drops off quickly with decreasing temperature. Therefore, CO mainly forms in the cold region in the vicinity of cooled walls of the cylinder. The smaller is the value of the maximal temperature during HCCI combustion, the vaster is the region with diminishing temperature near the wall of the piston (see Figure 12) and, as a consequence, the greater is the concentration of CO in the combustion exhaust. As a result, the major emission index of CO takes place upon production of SDO molecules and minor one in the basic case (see Table 2). However, the increase in the CO emission index is not very high and at conditions considered does not exceed a factor of 1.5. Upon the production of the equivalent amount of O atoms, this increase is certainly smaller and achieves a factor of 1.16 only, because of the greater temperature in the cylinder. The obtained results are related to the nominal regime of HCCI engine operation. However, it would be extremely fruitful to determine how the generation of the small amount of SDO in the thin layer placed near the cylinder head can influence on the operation of HCCI engine at low load. The concrete analysis was conducted for the operating regime with the smallest value of fuel-to-air equivalence ratio at which the HCCI engine with considered parameters could operate stably. Computations showed that such a value of φ at T0=471 K is equal to 0.17. In this case, the maximum mass-average temperature is reached at βm=11.25°. The production of 1% SDO molecules in the 5 mm


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thickness layer adjacent to the cylinder head in the time instant corresponding to the crank angle βp= -32° results in the earlier combustion of the charge (βm=3.5°). This provides the higher temperature and pressure in the cylinder during the compression-expansion stroke. This fact is illustrated by the traces of mass-average static temperature and volume-average static pressure shown in Figure 13. As is seen, upon the production of 1% SDO, the mass-average temperature achieves a value of ~1600 K, while, in the basic case (Tev)max=1380 K. The value of maximal pressure also increases notably from (Pev)max=43 atm in the basic case to (Pev)max=54 atm in the case of SDO production. The growth of pressure during the compression-expansion stroke due to the abundance of SDO in the region near the cylinder head results in the higher value of thermodynamic work (by 40%). This also follows from the P-V diagram depicted in Figure 14 for the cases considered. Shown in Figure 15 are the evolution of mass-average mass fractions of main species such as CH4, CO2 and H2O molecules and highly reactive radicals responsible for the chain mechanism development: OH, O and CH3, some important pollutants such as CO, unburnt hydrocarbons CnHm (CnHm=CH4+C2H6+C2H2+…), organics CxHyOz (CxHyOz=CH2O+CH3O+CH3O2+…) and nitrogen oxides NO, NO2, N2O. One can see that, in the basic case, the fuel (CH4) is not completely burnt and approximately 16% of CH4 remains in the combustion exhaust. Incomplete burning of the fuel at low equivalence ratio leads to the notable amounts of CO, CnHm and CxHyOz in the combustion products at EVO.31 The production of even small amount of SDO molecules (1% of total oxygen) in the thin layer adjacent to the cylinder head increases the combustion completeness (only 1.9% of CH4 remains being unburnt) and decreases the emissions of CO, CnHm and CxHyOz. In doing so the emissions of CO and unburnt hydrocarbons falls down by a factor of 2.5 and 9 respectively. Emission of organic compounds decreases by 23 times. It should be emphasized that, upon the combustion of such fuel-lean mixture (φ=0.17), emission


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of NO is extremely low (see Figure 15). This occurs because the combustion temperature is lower than the threshold value (~1800 K) for the formation of NO according to the thermal mechanism.29 It was shown earlier that at low combustion temperatures, N2O intermediate mechanism of NO formation became important both in gas turbine combustion32 and in HCCI engines33-34. On can see from Figure 15 that, at considered conditions, the most abundant specimen among N-containing compounds is N2O, but not the nitric oxide. N2O mass fraction is 2×10-6, that corresponds to the volume fraction of 1.3 ppm. It should note that the significant increase of N2O was detected experimentally35 in HCCI engine fueled with n-heptane when incomplete combustion occurs. N2O peak concentration of 1.7 ppm was observed when the combustion efficiency dropped to ~70%. Note that N2O is a very stable and strong infrared absorbing compound; its global warming potential is essentially higher than that of CO2, which makes N2O a problematic greenhouse gas. In the stratosphere, N2O can enhance ozone depletion due to the reaction of N2O with O3. The presence of 1% SDO in the thin layer located near the cylinder head leads to the reduction in the N2O concentration, though the emission index of NOx increases by a factor of 5 in this case, but its value is still smaller than that for N2O one. Thus, the production of even small amount of O2(a1∆g) molecules in a thin layer near the cylinder head at some optimal value of crank angle (βp= -32°) can be considered as an effective approach to controlling the ignition phasing and combustion enhancement in HCCI engine both at nominal and at low load regimes.

Conclusions CFD modeling was conducted to investigate the efficiency of the approach of the combustion enhancement and controlling the ignition timing in HCCI engine operating on methane-air mixture via the production of O2(a1∆g) molecules or O atoms in a thin layer adjacent to the cylinder head at some


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given value of crank angle. The computations showed that both the abundance of 1% O2(a1∆g) molecules in oxygen and the presence of the equivalent amount of O atoms (0.38%) in the layer of 5 mm thickness allowed one to accelerate the ignition both at nominal and at low load regimes. The production of O2(a1∆g) molecules is notably more effective for such a way of HCCI combustion enhancement. In both cases, the abundance of O2(a1∆g) molecules plays an important role in the intensification of chain reactions before the ignition event, though the contribution of the reactions with O2(a1∆g) molecules is much smaller in the case of O atom generation. Even upon the production of the small amount of SDO (~0.5% of total oxygen), the ignition occurs before the top-dead-center. In order to provide the maximal average values of temperature and pressure at the given value of the crank angle (βm=10°), it is needed to decrease the intake temperature of the charge. In the cases of the production of 1% SDO or the equivalent amount of atomic oxygen, this makes it possible to reduce notably the emission of NO (by a factor of 3 and 1.6, respectively) and, simultaneously, to increase slightly the value of mass specific power by 1.8% and 0.96%. At the same time, the emission of CO grows compared to the basic case by a factor of 1.2 and 1.01, respectively. It should be emphasized that the abundance of SDO or O atoms provides the smaller gradient of pressure and temperature in the cylinder during the compression-expansion stroke. This should result in the longer engine life and higher safety. At low load regime, the production of SDO in a thin layer, located near the cylinder head, allows one to extend the range of the stable operation of HCCI engine, to ensure the increase of engine power at given power setting (by a factor of 1.4) and to reduce substantially the emission of CO, unburnt hydrocarbons, and organic compounds. Thus, the approach, based on the photochemical activation of O2 molecules is a thin layer adjacent to the cylinder head with a rather small input energy, may be very effective to control the ignition timing and to enhance the combustion in HCCI engine.

Acknowledgments 15 ACS Paragon Plus Environment

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This work was supported by the Russian Foundation for Basic research (grants № 17-01-00810, 17-08-01423) and by the Council of President of Russian Federation for support of Young Russian Scientists and Leading Scientific Schools (grant № SS-7018.2016.8).


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Turbulence and External Exhaust Gas Recirculation on HCCI Combustion Mode and Exhaust Emissions. Energy Fuels 2009, 23(9), 4295-4303. (5)

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Conventional and Low Temperature Combustion Conditions. Energy Fuels 2013, 27(12), 7827-7842. (6)

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mixture with ozone addition. J. Propul. Power 2006, 22, 151-157. (10) Foucher, F.; Higelin, P.; Mounaïm-Rousselle, C.; Dagaut, P. Influence of ozone on the combustion of n-heptane in a HCCI engine. Proc. Combust. Inst. 2013, 34, 3005-3012. 17 ACS Paragon Plus Environment

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(11) Masurier, J.-B.; Foucher, F.; Dayma, G.; Dagaut, P. Homogeneous charge compression ignition combustion of primary reference fuels influenced by ozone addition. Energy Fuels 2013, 27 (9), 54955505. (12) Masurier, J.-B.; Foucher, F.; Dayma, G.; Dagaut, P. Investigation of iso-octane combustion in a homogeneous charge compression ignition engine seeded by ozone, nitric oxide and nitrogen dioxide. Proc. Combust. Inst. 2015, 35(3), 3125-3132. (13) Starik, A. M.; Kozlov, V. E.; Titova, N. S. On the influence of singlet oxygen molecules on characteristics of HCCI combustion: A numerical study. Combust. Theory Model. 2013, 17, 579-609. (14) Starik, A. M.; Kozlov, V. E.; Titova, N. S. Modeling study of the possibility of HCCI combustion improvement via photochemical activation of oxygen molecules. Energy Fuels 2014, 28, 2170-2178. (15) Dubreuil, A.; Foucher, F.; Mounaïm-Rousselle, C.; Dayma, G. ; Dagaut, P. HCCI combustion: effect of NO in EGR. Proc. Combust. Inst. 2007, 31, 2879-2886. (16) Andrae, J. C. G. Kinetic modeling of the influence of NO on the combustion phasing of gasoline surrogate fuels in an HCCI engine. Energy Fuels 2013, 27(11), 7098-7107. (17) Contino, F.; Foucher, F.; Dagaut, P.; Lucchini, T.; D’Errico, G.; Mounaïm-Rousselle, C. Experimental and numerical analysis of nitric oxide effect on the ignition of iso-octane in a single cylinder HCCI engine. Combust. Flame 2013, 160, 1476-1483. (18) Lavid, M.; Nachshon, Y.; Gulati, S. K.; Stevense J. G. Photochemical ignition of premixed hydrogen/oxygen mixtures with ArF laser. Combust. Sci. Technol. 1994, 96(4), 231-245. (19) Brainin, B. I.; Volkov, S. Yu.; Kobtsev, V. D.; Kozlov, D. N.; Kostritsa, S. A.; Pelevkin, A. V.; Smirnov, V. V.; Stelmakh, O. M.; Titova, N. S.; Torokhov, S. A.; Starik A. M. Ignition and combustion of H2-O2 mixture upon dissociation of O2 molecules by laser radiation with 193-nanometer wavelength. In Nonequilibrium processes in physics and chemistry. Volume 1. Plasma, clusters, and atmosphere; Starik, A.M., Frolov, S.M., Eds.; TORUS PRESS:Moscow, 2016, pp 32-41. 18 ACS Paragon Plus Environment

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(20) ANSYS FLUENT. Version 12. ANSYS Inc.; January 2009. (21) Joelsson, T. Large eddy simulation of turbulent reactive flows under HCCI engine conditions. Thesis for the Degree of Doctor of Philosophy in Engineering. Lund, SWEDEN:Division of Fluid Mechanics, Department of Energy Sciences, Faculty of EngineeringeLTH, Lund University, 2011. (22) Kozlov, D. N.; Kobtsev, V. D.; Stel’makh, O. M.; Smirnov, V. V. Study of collisional deactivation of O2(b1Σg+) molecules in a hydrogen-oxygen mixture at high temperatures using laserinduced gratings. J. Experimental and Theoretical Physics 2013, 117(1), 36-47. (23) Eisenberg, W. C.; Snelson, A.; Butler, R.; et al. Gas phase generation of O2(+∆g) at atmospheric pressure by direct laser excitation. J. Photochemistry 1984, 25(2-4), 439-448. (24) Sharipov, A. S.; Starik, A. M. Analysis of the reaction and quenching channels in a H+O2(a1∆g) system. Phys. Scr. 2013, 88, 058305. (25) Lebedev, A. V.; Deminsky, M. A.; Zaitzevsky, A. V.; Potapkin, B. V. Effect of O2(a1∆g) on the low-temperature mechanism of CH4 oxidation. Combust. Flame 2013, 160(3), 530-538. (26) Vagin, N. P.; Kochetov, I. V.; Napartovich, A. P.; Yuryshev, N. N. Acceleration of methaneoxygen mixture ignition by adding singlet oxygen produced in a chemical generator. Bulletin of the Lebedev Physics Institute 2016, 43(7), 211-216. (27) Westbrook, C. K. Chemical kinetics of hydrocarbon ignition in practical combustion systems. Proc. Combust. Inst. 2000, 28(2), 1563-1577. (28) Aceves, S. M.; Flowers, D.; Westbrook, C. K.; Smith, J. R.; Pitz, W.; Dibble, R.; Christensen, M.; Johansson, B. A multi-zone model for prediction of HCCI combustion and emissions. Society of Automotive Engineers, SAE Technical Paper Series 2000-01-0327, 2000. (29) Zeldovich, Y. B. The oxidation of nitrogen in combustion and explosions. Acta Physicochimica USSR 1946. (30) Sjöberg, M.; Dec, J. E. An investigation into lowest acceptable combustion temperatures for 19 ACS Paragon Plus Environment

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hydrocarbon fuels in HCCI engines. Proc. Combust. Inst. 2005, 30, 2719-2726. (31) Saxena, S.; Bedoya, I. D. Fundamental phenomena affecting low temperature combustion and HCCI engines, high load limits and strategies for extending these limits. Prog. Energy Combust. Sci. 2013, 39, 457-488. (32) Lentini, D. Prediction of NOx emissions in gas turbine combustors inclusive of the N2O contribution. Proc. Inst. Mech. Eng., Part A 2003, 217, 83-90. (33) Amnéus, P.; Mauss, F.; Kraft, M.; Vressner, A.; Johansson, B. NOx and N2O formation in HCCI engine. SAE Paper No. 2005-01-0126, 2005. (34) Starik, A. M., Korobov, A. N., Titova N. S. Combustion improvement in HCCI engine operating on synthesis gas via addition of ozone or excited oxygen molecules to the charge: Modeling study. Int. J. Hydrogen Energy 2017, 42, http://dx.doi.org/10.1016/j.ijhydene.2017.01.179. (35) Li, H.; Neill, W. S.; Guo, H.; Chippior, W. The NOx and N2O emission characteristics of an HCCI engine operated with n-heptane. J. Energy Resources Tech. 2012, 134(1), 011101.


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Table 1. Engine parameters and operating conditions for basic regime in HCCI engine. Engine speed, rpm Fuel Basic equivalence ratio, φ Cylinder bore diameter, cm Cylinder clearance volume, cm3 Crank radius, cm Connecting rod length, cm Initial swirl ratio Inlet valve closing, degree Exhaust valve opening, degree T0, K P0, atm

1000 CH4 0.3846 10 67.3184 6.64 23.24 3 -142 115 471 1.065

Table 2. Maximal values of mass-average static temperature (Tev)max and volume-average static pressure (Pev)max; crank angle βm at which (Tev)max is realized; maximal value of local static temperature Tmax; maximal values of pressure gradient (d(Pev)/dt)max; fuel mass in cylinder mfuel; cycle work W; specific cycle work W/mfuel; the change in the specific cycle work ∆(W/mfuel); energy input to produce SDO or O atoms Ein; specific energy input to produce SDO or O atoms Ein/mfuel; mass fractions of NO, NO2 and CO, as well as their emission indices EINO, EINOx, EICO in the combustion exhaust for cases considered at φ=0.3846. 471 K 0% (Tev)max, K 2024 9.25 βm, degree Tmax, K 2206 (Pev)max, bar 64.9 (d(Pev)/dt)max, bar/s 1.02e10 mfuel, kg 1.76e-5 W, J 386 W/mfuel, J/kg 2.19e7 ∆(W/mfuel), J/kg ∆(W/mfuel), % Ein, J Ein/mfuel, J/kg CNO 7.89e-4 CNO2 1.27e-4 CCO 2.16e-4 EINO, g/(kg fuel) 36 EINOx, g/(kg fuel) 61 EICO, g/(kg fuel) 9.8

0.5%SDO 1%SDO 2155 0.75 2291 75.4 9.8e9 1.76e-5 371 2.11e7 -8.7e5 -4

2178 -1.125 2330 76.2 9.7e9 1.76e-5 363 2.06e7 -1.3e6 -5.9

2.41e-3 1.94e-4 2.80e-5 110 177 1.3

3.16e-3 2.44e-4 2.58e-5 144 231 1.2

1%SDO, eq O, eqO eqO 431 K 452 K (0.5%SDO) (1%SDO) 2124 2112 1944 1993 5 4.125 10.5 9.625 2258 2244 2134 2177 73.4 72.1 66.4 66.5 8.6e9 8.8e9 3.7e9 6.4e9 1.76e-5 1.93e-5 1.84e-5 1.76e-5 380 378 431 407 2.15e7 2.16e7 2.23e7 2.21e7 -4.6e5 -3.3e5 4.01e5 2.11e5 -2.1 -1.6 1.8 0.96 1.39 1.32 7.2e4 7.2e4 1.72e-3 2.65e-4 4.96e-4 1.52e-3 1.3e-4 1.1e-4 6.77e-5 1.13e-4 2.46e-5 2.87e-5 2.65e-4 2.47e-4 69 78 12.1 22.6 112 126 21.6 39.8 1.3 1.1 12 9.9


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Table 3. Maximal values of mass-average static temperature (Tev)max and volume-average static pressure (Pev)max; maximal value of local static temperature Tmax; cycle work W; as well as emission indices EI of CO, NOx, N2O, CxHy and CxHyOz in the combustion exhaust. φ

Basic case 1% SDO

(Tev)max, (Pev)max, Tmax, W, J K atm K 0.17 1378 43 1621 104.6

CO 350

EI, g/(kg fuel) NOx N2O CxHy CxHyOz 0.026 0.21 180 59

0.17

142

0.14

1594

54

1725 145.2


0.17

21

2.5

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laser radiation

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cylinder head

irradiation region, thickness a=5 mm

piston

β

Figure 1. The schematic of the engine cylinder with the region exposed to laser radiation. Tev, K

Pev, Pa

2000 6

6.0x10 1500

6

without active species, 471K 0.5% SDO, 471K equiv. O, 471K 1% SDO, 471K equiv. O, 471K 1% SDO, 431K equiv. O, 452K

1000

500 -40

-20

0 β, degree

20

40

4.0x10

6

2.0x10

0.0 -40

-20

0

β, degree

20

40

Figure 2. Evolution of mass-average static temperature Tev and volume-average static pressure Pev in the engine cylinder in the basic case without active species (black solid curve), in the cases of production of 0.5 and 1% SDO in total oxygen (dotted and dashed blue curves) or equivalent amount of atomic oxygen (dotted and dashed red curves) at T0=471 K and in the cases of production of 1% SDO in total oxygen (solid blue curve) at T0=431 K or equivalent amount of atomic oxygen at T0=452 K (solid red curve). a

b


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Pev, Pa

Pev, Pa

1% SDO, 471 K equiv. O, 471 K without active species, 471 K

6

6.0x10

Page 24 of 30

without active species, 471 K

6

6.0x10

1% SDO, 431 K equiv. O, 452 K

6

6

4.0x10

6

2.0x10

4.0x10

6

2.0x10

-5

6.0x10

-4

1.0x10

-4

1.4x10

3

V, m

-5

6.0x10

-4

1.0x10

-4

1.4x10

3

V, m

Figure 3. P-V diagram in the vicinity of TDC for HCCI cycle in the basic case without active species (black solid curve) and in the cases of the production of 1% SDO in total oxygen or equivalent amount of O atoms at T0=471 K (blue and red dashed curves) (a) as well as in the cases of production of 1% SDO at T0=431 K (blue solid curve) or equivalent amount of O atoms at T0=452 K (red solid curve) (b). β=0°

β=1.5°

β=2.5°

β=3°

β=4°

β=5°

β=6°

Figure 4. Temperature fields in HCCI engine at different crank angles in the basic case (without active species) and T0=471 K. Half of the cylinder is shown in the images. 24 ACS Paragon Plus Environment

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β=0°

β=1.5°

β=2.5°

β=3°

β=4°

β=5°

β=6°

Figure 5. Temperature fields in HCCI engine in the case of 1% SDO production in the layer located near the cylinder head at βp~-32° and T0=431 K. Half of the cylinder is shown in the images.


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β=0°

β=1.5°

β=2.5°

β=3°

β=4°

β=5°

β=6°

Figure 6. Temperature fields in HCCI engine in the case of the production of equivalent amount of O atoms in the layer located near the cylinder head at βp~-32° and T0=452 K. Half of the cylinder is shown in the images. Ci

OH

-4

CO2

Ci

H2O H2O

10

O

CH3

-3

10

-6

10

1

O2(a ∆g)

OH -8

CH3

O

10

H2O2 -7

-10

10

-40

CO2

-5

10

-20

0 β, degree

20

40

10

-40

-20

0 β, degree

20

40

Figure 7. Mass-average mass fractions Ci of main species as a function of crank angle β for the basic case (T0=471 K) and for the cases of production of 1% SDO (T0=431 K) or equivalent amount of O 26 ACS Paragon Plus Environment

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atoms (T0=452 K) (solid, dotted and dashed curves, respectively). a

b

c

Figure 8. Fields of O atom mass fraction in HCCI engine at TDC (β=0°) in the basic case at T0=471 K (a) as well as in the cases of production of 1% SDO at T0=431 K (b) or equivalent amount of O atoms at T0=452 K (c). Half of the cylinder is shown in the images. a

b

c

Figure 9. Static temperature fields at EVO in the cylinder of HCCI engine operating on methane for the basic case (T0=471 K) (a) and upon the production of 1% SDO (T0=431 K) (b) or equivalent amount of O atoms (T0=452 K) (c). Half of the cylinder is shown in the images.


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a

b

Page 28 of 30

c

Figure 10. NO mass fraction fields at EVO in the cylinder of HCCI engine operating on methane for the basic case (T0=471 K) (a) and upon the production of 1% SDO (T0=431 K) (b) or equivalent amount of O atoms (T0=452 K) (c). Half of the cylinder is shown in the images.

Ci

NO

-4

10

NO2 -6

N2O

10

N2O -8

10

-10

10

-40

-20

0 β, degree

20

40

Figure 11. Mass-average mass fractions Ci of NO, NO2 and N2O as a function of crank angle β for the basic case (T0=471 K) and upon the production of 1% SDO (T0=431 K) or equivalent amount of O atoms (T0=452 K) (solid, dotted and dashed curves, respectively).


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a

b

c

Figure 12. CO mass fraction fields at EVO in the cylinder of HCCI engine operating on methane for the basic case (T0=471 K) (a) and upon the production of 1% SDO (T0=431 K) (b) or equivalent amount of O atoms (T0=452 K) (c). Half of the cylinder is shown in the images. Tev, K

Pev, Pa

1600

6

5x10 1400

1% SDO

6

4x10

0% SDO

1200

6

3x10

0% SDO 1000

6

2x10

800 600 -40

1% SDO

6

1x10 -20

0 β, degree

20

40

0 -40

-20

0

20

40

β, degree

Figure 13. Evolution of mass-average static temperature Tev and volume-average static pressure Pev vs. crank angle at low load regime with φ=0.17 for the basic case (without SDO production) and upon production of 1% SDO in the thin layer (solid and dashed curves, respectively).


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Pev, Pa

1% SDO

6

4.0x10

0% SDO

6

2.0x10

-5

6.0x10

1.0x10

-4

1.4x10

-4

3

V, m

Figure 14. P-V diagram for the low load regime with φ=0.17 for the basic case (without SDO production) and upon production of 1% SDO (solid and dotted curves, respectively). Ci

Ci

CO2

0.020

H2O

OH

-5

10

CH3

CO2

0.015

-6

10 0.010

CH4

O

-7

10

0.005

O -8

0.000 -40

Ci

-20

0

β, degree

40

10

-40

-20

0 β, degree

Ci

CO

CxHy

0.008

20

20

40

N2O

-6

10

NO

0.006 NO2

-7

0.004

CxHyOz*10

CO CxHy

0.002 0.000 -40

10

NO

-8

10

-9

-20

0

20

40

β, degree

60

80

10

-40

-20

0

20 40 β, degree

60

80

Figure 15. Evolution of mass-average mass fractions Ci of main species, highly reactive radicals and pollutants as a function of crank angle at low load regime with φ=0.17 for the basic case (solid curves) and upon production of 1% SDO in the thin layer (dotted curves).


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