Numerical Study of the Influence of the Photochemical Activation of

Article pubs.acs.org/EF

Numerical Study of the Influence of the Photochemical Activation of Oxygen Molecules on Homogeneous Charge Compression Ignition Performance A. M. Starik,* V. E. Kozlov, and N. S. Titova Central Institute of Aviation Motors, Scientific Educational Centre of Physicochemical Kinetics and Combustion, Moscow 111116, Russia S Supporting Information *

ABSTRACT: Numerical analysis of the possibility of the combustion enhancement in a homogeneous charge compression ignition (HCCI) engine operating on methane via the production of O2(a1Δg) molecules or O atoms by resonance laser radiation in the 5 mm thickness layer, located near the cylinder head, is conducted for both the nominal- and low-load regimes. It is shown that the abundance of both singlet delta oxygen and atomic oxygen in such a layer substantially accelerates 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(a1Δg) molecules or O atoms. In doing so, the production of O2(a1Δg) molecules is more effective for the combustion enhancement in a HCCI engine. At a low-load regime, the production of O2(a1Δg) 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 a few times the emissions of CO, unburnt hydrocarbons, and organic compounds.

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the charge.9−17 In particular, it has been shown that the oxidation of the fuel−air mixture occurs faster under ozoneseeded 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 delta state O2(a1Δg) (SDO) can affect the ignition timing and combustion performance in a HCCI engine operating on methane. Moreover, this allows for 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 have 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 a 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 a1Δg 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 the zero-dimensional (0D) single-zone model, which assumes that O2(a1Δg) molecules are produced in the whole volume of the fuel−air mixture. In reality, it is more reasonable to produce O2(a1Δg)

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 known, HCCI combustion is based on the compression of a well-mixed fuel-lean mixture to a 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 (UHCs) at a 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 used. 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. 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 a HCCI engine, which was widely investigated for the past few years, is the admixture of small amounts of chemically active species to © 2017 American Chemical Society

Received: January 30, 2017 Revised: June 28, 2017 Published: June 30, 2017 8608

DOI: 10.1021/acs.energyfuels.7b00305 Energy Fuels 2017, 31, 8608−8618

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Energy & Fuels

dye, and solid laser with an Al2O3Ti2+ crystal with optical pumping.22 It was also shown previously23 that the O2(a1Δg) molecule could be produced through the exposure of air by the radiation of a Nd:YAG laser with 1.065 μm wavelength. Oxygen atoms can be produced by the radiation with 193.3 nm generated by an excimer ArF laser.18 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(a1Δg) 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 a moderate value of the radiation intensity 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 a given amount of O2(a1Δg) molecules. Note that, as a result of the fact that the energy required for the production of an O atom is greater than that needed for the excitation of an O2 molecule to the a1Δg state, the concentration of O atoms in the irradiated region is smaller compared to the concentration of O2(a1Δg) at an identical input energy of laser radiation. Shown in Figure 1 is the schematic of the HCCI cylinder with the laser system for the exposure of the mixture to laser photons.

molecules in a thin layer of the 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 singlezone thermochemical model cannot be used, and analysis should be performed on the basis of two-dimensional (2D) computational fluid dynamics (CFD) calculations. Besides the activation of O2 molecules via their excitation to the a1Δg 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 photodissociation 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 a HCCI engine upon the photochemical activation of O2 molecules in a thin layer of the 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 using the 2D axisymmetric CFD model based on Favre-averaged non-stationary Navier−Stokes equations for the reacting mixture including excited O2(a1Δg) 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 the HCCI engine were chosen the same as reported in the guideline of the ANSYS program package and presented in Table 1. EGR in the cylinder was

Table 1. Engine Parameters and Operating Conditions for the Basic Regime in a 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 (deg) exhaust valve opening (deg) T0 (K) P0 (atm)

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

Figure 1. Schematic of the engine cylinder with the region exposed to laser radiation.

For the description of ignition and combustion in a HCCI engine, the reaction mechanism developed previously for the CH4−O2−N2 mixture and comprising the processes with electronically excited O2(a1Δg) 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(a1Δg) and CH3 + O2(a1Δg) 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 the Supporting Information. In addition, as was recently shown,26 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 the CH4−O2 mixture upon the injection of O2(a1Δg) molecules at the entrance of the reactor, produced via chemical reaction

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 a1Δg 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(a1Δg) molecules can be generated as a result of laser-induced excitation of ground-state O2(X3Σg−) to the singlet sigma state O2(b1Σg+) by the radiation with 762.3 nm wavelength and 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,

Cl 2 + 2KOH + H 2O2 → O2 (a1Δg ) + 2KCl + 2H 2O 8609


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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 (solid black curve), in the cases of production of 0.5 and 1% SDO in total oxygen (dotted and dashed blue curves) or an 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 an equivalent amount of atomic oxygen at T0 = 452 K (solid red curve).

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

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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 the 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 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°. Because the (Pev)max value is implemented before TDC, the essential energy must be spent at the compression stroke 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 a conclusion can also be 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 a HCCI cycle is determined by the

RESULTS AND DISCUSSION Consider, at first, how the production of SDO in a thin layer near the cylinder head influences ignition phasing in a 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% SDO in total oxygen (herein and hereafter the mole fraction of SDO in total oxygen is indicated) and upon the production of an 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 basis of 0D simulation13,14 that such a 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 the concentration of active species (this corresponds to the greater energy of laser radiation put into the gas), the earlier the ignition event in the 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 predicted earlier on the basis 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 are no experimental data on the influence of SDO molecules or O atoms on combustion phasing in a HCCI engine. For the HCCI engine operating on 8610


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Table 2. Maximal Values of the 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 the Local Static Temperature Tmax, Maximal Values of the Pressure Gradient (dPev/dt)max, Fuel Mass in the Cylinder mfuel, Cycle Work W, Specific Cycle Work W/mfuel, Change in the Specific Cycle Work Δ(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, and Their Emission Indices EINO, EINOx, and EICO in the Combustion Exhaust for Cases Considered at ϕ = 0.3846 471 K 0%

0.5% SDO

1% SDO

equiv O (0.5% SDO)

equiv O (1% SDO)

1% SDO, 431 K

equiv O, 452 K

(Tev)max (K) βm (deg) Tmax (K) (Pev)max (bar) (dPev/dt)max (bar/s) mfuel (kg) W (J) W/mfuel (J/kg) Δ(W/mfuel) (J/kg) Δ(W/mfuel) (%) Ein (J) Ein/mfuel (J/kg) CNO CNO2

2024 9.25 2206 64.9 1.02 × 1010 1.76 × 10−5 386 2.19 × 107

2155 0.75 2291 75.4 9.8 × 109 1.76 × 10−5 371 2.11 × 107 −8.7 × 105 −4

2178 −1.125 2330 76.2 9.7 × 109 1.76 × 10−5 363 2.06 × 107 −1.3 × 106 −5.9

2112 5 2244 72.1 8.6 × 109 1.76 × 10−5 380 2.16 × 107 −3.3 × 105 −1.6

2124 4.125 2258 73.4 8.8 × 109 1.76 × 10−5 378 2.15 × 107 −4.6 × 105 −2.1

7.89 × 10−4 1.27 × 10−4

2.41 × 10−3 1.94 × 10−4

3.16 × 10−3 2.44 × 10−4

1.52 × 10−3 1.1 × 10−4

1.72 × 10−3 1.3 × 10−4

1944 10.5 2134 66.4 3.7 × 109 1.93 × 10−5 431 2.23 × 107 4.01 × 105 1.8 1.39 7.2 × 104 2.65 × 10−4 6.77 × 10−5

1993 9.625 2177 66.5 6.4 × 109 1.84 × 10−5 407 2.21 × 107 2.11 × 105 0.96 1.32 7.2 × 104 4.96 × 10−4 1.13 × 10−4

CCO EINO (g/kg of fuel) EINOx (g/kg of fuel) EICO (g/kg of fuel)

2.16 × 10−4 36 61 9.8

2.80 × 10−5 110 177 1.3

2.58 × 10−5 144 231 1.2

2.87 × 10−5 69 112 1.3

2.46 × 10−5 78 126 1.1

2.65 × 10−4 12.1 21.6 12

2.47 × 10−4 22.6 39.8 9.9

volume available for combustion in the cylinder at the crank angles β = −142° and 115°, respectively. In the case of 1% SDO content, the area of the P(V) figure is smaller than that in the basic case. Acceleration of ignition as a result of the production of active species (SDO or atomic oxygen) makes it possible to decrease the initial temperature of the charge in order to realize the (Tev)max value at the crank angle that is identical to crank angle 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 an 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 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 the temperature and pressure during the compression−expansion stroke also changes compared to the basic case. Therefore, the abundance of SDO or atomic oxygen in the thin layer in the region near the cylinder head provides the greater 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.

Figure 4. Temperature fields in a 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.

area of the figure bounded by the line P(V) and calculated by the formula Ea = ∫ VV10P dV, where V0 and V1 are the values of the 8611


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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.

Figure 6. Temperature fields in HCCI engine in the case of the production of an 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.

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 a 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 the temperature fields in the cylinder of a 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 the 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 the maximal temperature located in the central region (see Figure 4). This region spreads to the mixture layers with a 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 a temperature of 1600 K is equal to ∼50 m/s in the cylinder axis direction and ∼150 m/s in the radial direction. Because the flame speed in the 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 that 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, which are appreciably smaller than those in the basic case. The decrease in 8612


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Figure 7. Mass-average mass fractions Ci of main species as a function of the crank angle β for the basic case (T0 = 471 K) and for the cases of production of 1% SDO (T0 = 431 K) or an equivalent amount of O atoms (T0 = 452 K) (solid, dotted, and dashed curves, respectively).

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

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

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

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

temperature field over the piston is less uniform than that 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

“visible speed” of the hot region boundary is explained by the smaller value of the temperature realized at TDC (by ∼30 K) as a result of 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 8613


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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 whose concentration is higher at TDC in the case of SDO molecule or O atom production (by 50 and 25%, respectively) is H2O2. This molecule, which is rather stable at a low temperature, dissociates intensively at T > 1000 K (such a temperature is realized at the end of the 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 for the ignition in a 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, comparing the fields of the 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 the 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 to the basic case. This domain adjoins to the layer where active species form and shift closer to the sidewall as a result of 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 to Figures 5 and 6). The precisely higher local concentration of active species provides the mixture ignition at some lower initial temperature, when SDO or O atom are produced and, as a consequence, the achievement of the maximal temperature value at the same crank angle (βm ∼ 10°) as 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) mechanism,29 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

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

= 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. To clarify the reason for the acceleration of ignition and combustion as a result of the generation of SDO molecules or O atoms in the mixture layer adjacent to the cylinder head at 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 the 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(a1Δg) molecules: O + O + M = O2(a1Δg) + M, with the higher concentration than those of O, H, and OH. The further decrease of the SDO molecule concentration is due to the chemical reactions with participation of SDO: CH4 + O2(a1Δg) = CH3 + HO2, CH3 + O2(a1Δg) = CH2O + OH, CH3 + O2(a1Δg) = CH3O2, and CH3 + O2(a1Δg) = CH3O + O, as well as its collisional quenching. In the case of direct production of SDO, its concentration is by a factor of 20 greater than that upon the production of O atoms. The concentrations of active radicals increase more regularly (without sharp peaks at β ∼ −32°) and are notably higher than those in the case of O atom generation.

Table 3. Maximal Values of Mass-Average Static Temperature (Tev)max and Volume-Average Static Pressure (Pev)max, Maximal Value of the Local Static Temperature Tmax, Cycle Work W, and Emission Indices EI of CO, NOx, N2O, CxHy, and CxHyOz in the Combustion Exhaust EI (g/kg of fuel) basic case 1% SDO

ϕ

(Tev)max (K)

(Pev)max (atm)

Tmax (K)

W (J)

CO

NOx

N2O

CxHy

CxHyOz

0.17 0.17

1378 1594

43 54

1621 1725

104.6 145.2

350 142

0.026 0.14

0.21 0.17

180 21

59 2.5

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

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 the operation of a HCCI engine at low load. The concrete analysis was conducted for the operating regime with the smallest value of fuel/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 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 seen upon the production of 1% SDO, the massaverage 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 as a result of 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, such as OH, O and CH3, and some important pollutants, such as CO, UHCs CnHm (CnHm = CH4 + C2H6 + C2H2 + ...), organics CxHyOz (CxHyOz = CH2O + CH3O + CH3O2 + ...), and nitrogen oxides NO, NO2, and 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 a low equivalence ratio leads to notable amounts of CO, CnHm, and CxHyOz in the combustion products at EVO.31 The production of even a small amount of SDO molecules (1% of total oxygen) in the thin layer adjacent to the cylinder head

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

fuel; the emission index of NOx is calculated by the formula EINOx = EINO × μNO2/μNO + EINO2, where μNO and μNO2 are the molar masses of NO and NO2, respectively. 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 the decrease of temperature. Therefore, CO mainly forms in the cold region in the vicinity of cooled walls of the cylinder. The smaller the value of the maximal temperature during HCCI combustion, the vaster the region with a diminishing temperature near the wall of the piston (see Figure 12) and, as a consequence, the greater the concentration of CO in the combustion exhaust. As a result, the major emission index of CO takes place upon the production of SDO molecules and the minor emission index of CO takes place in the basic case (see Table 3). 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 8615


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Energy & Fuels

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

although the emission index of NOx increases by a factor of 5 in this case, its value is still smaller than that of N2O. Thus, the production of even a small amount of O2(a1Δg) molecules in a thin layer near the cylinder head at some optimal value of the crank angle (βp = −32°) can be considered as an effective approach to controlling the ignition phasing and combustion enhancement in a HCCI engine at both nominaland low-load regimes.

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 UHCs fall by a factor of 2.5 and 9, respectively. The emission of organic compounds decreases by 23 times. It should be emphasized that, upon the combustion of such a fuel-lean mixture (ϕ = 0.17), emission 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, the N2O intermediate mechanism of NO formation became important in both gas turbine combustion32 and HCCI engines.33,34 One can see from Figure 15 that, at considered conditions, the most abundant specimen among N-containing compounds is N2O but not nitric oxide. The N2O mass fraction is 2 × 10−6, which corresponds to the volume fraction of 1.3 ppm. It should note that the significant increase of N2O was detected experimentally35 in a HCCI engine fueled with nheptane when incomplete combustion occurs. The 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 as a result of the reaction of N2O with O3. The presence of 1% SDO in the thin layer located near the cylinder head leads to reduction in the N2O concentration;

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CONCLUSION CFD modeling was conducted to investigate the efficiency of the approach of the combustion enhancement and controlling the ignition timing in a HCCI engine operating on a methane− air mixture via the production of O2(a1Δg) molecules or O atoms in a thin layer adjacent to the cylinder head at some given value of the crank angle. The computations showed that both the abundance of 1% O2(a1Δg) 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 at both nominal- and low-load regimes. The production of O2(a1Δg) molecules is notably more effective for such a way of HCCI combustion enhancement. In both cases, the abundance of O2(a1Δg) molecules plays an important role in the intensification of chain reactions before the ignition event, although the contribution of the reactions with O2(a1Δg) molecules is much smaller in the case of O atom generation. Even upon the production of a small amount of SDO (∼0.5% of total oxygen), the ignition occurs before the TDC. To 8616


Energy & Fuels provide the maximal average values of the temperature and pressure at the given value of the crank angle (βm = 10°), the intake temperature of the charge needs to decrease. 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%, respectively. 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 a smaller gradient of pressure and temperature in the cylinder during the compression−expansion stroke. This should result in a longer engine life and higher safety. At a 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 a HCCI engine, to ensure the increase of engine power at a given power setting (by a factor of 1.4), and to reduce substantially the emissions of CO, UHCs, and organic compounds. Thus, the approach based on the photochemical activation of O2 molecules in a thin layer adjacent to the cylinder head with a rather small input energy may be very effective to control the ignition timing and enhance the combustion in a HCCI engine.

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ACKNOWLEDGMENTS

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REFERENCES

This work was supported by the Russian Foundation for Basic Research (Grants 17-01-00810 and 17-08-01423) and the Council of the President of the Russian Federation for support of Young Russian Scientists and Leading Scientific Schools (Grant SS-7018.2016.8).

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.7b00305. Induction delay time τin as a function of the initial temperature T0 in the mixture CH4/O2/Ar at ϕ = 0.5 and P0 = 5, 10, and 20 bar (Figure S1), ignition delay time τin as a function of the initial temperature T0 during the CH4−C2H6−C3H8 = 90:6.6:3.3 mixture oxidation in air at ϕ = (a) 0.5 and (b) 1 and P0 = 30 atm (Figure S2), calculated and measured spatial profiles of the OH mole fraction in the CH4−air laminar flame at T0 = 300 K, P = 125 Torr, and ϕ = 0.9 (Figure S3), predicted (lines) and measured (symbols) values of flame speed Un as a function of ϕ for the CH4−air mixture at P0 = 1 atm and T0 = 300 K (Figure S4), spatial profiles of O2, CH4, H2O, and CO2 as well as CO and H2 species mole fractions and temperature in the flame front for the CH4−air mixture with ϕ = 1 (Figure S5), calculated and measured spatial profiles of the NO concentration in the atmospheric CH4−air flame with heat losses and corresponding temperature profiles for ϕ = 0.75 and 1 (Figure S6), and dependence of the ignition delay length Lin on the mole fraction of the O2(a1Δg) molecules in the total molecular oxygen in the H2/O2 = 5:2 mixture at the pressure P0 = 10 Torr and flow velocity u0 = 17 m/s (Figure S7) (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest. 8617


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Numerical Study of the Influence of the Photochemical Activation of

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