Combustion improvement in HCCI engine operating on synthesis gas via addition of ozone or excited oxygen molecules to the charge: Modeling study

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Combustion improvement in HCCI engine operating on synthesis gas via addition of ozone or excited oxygen molecules to the charge: Modeling study A.M. Starik a,b,*, A.N. Korobov a,b, N.S. Titova a a

Central Institute of Aviation Motors, Moscow, 111116, Russia Moscow Institute of Physics and Technology (State University), Department of Aeromechanics and Flight Engineering, Moscow Region, Zhukovsky, 140180, Russia

b

article info

abstract

Article history:

Comprehensive analysis of the possibility of the improvement of combustion and reduc-

Received 9 November 2016

tion of NO and CO emissions for HCCI engine operating on syngaseair mixture through the

Received in revised form

production in the intake charge highly reactive species, such as singlet delta oxygen

26 January 2017

O2(a1Dg) or ozone molecules, is conducted. The computations with the usage of detailed

Accepted 27 January 2017

reaction mechanism of syngas oxidation, involving the reactions with O2(a1Dg) and O3

Available online xxx

molecules, have showed that the abundance of O2(a1Dg) or O3 molecules in the charge accelerated the ignition and made it possible to increase slightly the mass specific energy

Keywords:

released during HCCI combustion and to reduce simultaneously the NO and CO emissions

HCCI

both at nominal and at low load regimes. Moreover, the production of O2(a1Dg) or O3

Syngas

molecules in the charge even in a small amount allows one to extend the range of stable

Ozone

combustion toward the extremely fuelelean mixture and ensure the stable operation of

Singlet delta oxygen

HCCI engine at low load. The advance in the energy released during combustion due to the

Combustion enhancement

presence of O2(a1Dg) or O3 in the charge is substantially greater the energy needed for the

Emissions

production of O2(a1Dg) and O3 molecules. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.

Introduction Nowadays, a great interest of researchers and practical engineers is expressed to the development of technologies aimed at the enhancement of ignition and combustion in the internal combustion engines (IC) [1e7]. Among such engines, the devices using the homogeneous charge compression ignition

(HCCI) mode are considered as very promising ones. A number of studies demonstrated that HCCI engines had significant advantages against conventional IC engines with spark ignition or diesel engines. They possess improved emission characteristics: smaller emissions of NO and CO and the absence of soot particles in the combustion exhaust. Partly, the results of the researches of the features of HCCI combustion were summarized in the overviews [1,2].

* Corresponding author. Central Institute of Aviation Motors, Moscow, 111116, Russia. E-mail address: [email protected] (A.M. Starik). http://dx.doi.org/10.1016/j.ijhydene.2017.01.179 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. Please cite this article in press as: Starik AM, et al., Combustion improvement in HCCI engine operating on synthesis gas via addition of ozone or excited oxygen molecules to the charge: Modeling study, International Journal of Hydrogen Energy (2017), http://dx.doi.org/ 10.1016/j.ijhydene.2017.01.179

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However, HCCI engines have some disadvantages. They operate stably in a narrow range of power setting and produce smaller output energy than that of traditional IC engines with spark plug. The other important issue arising for such engines is the necessity of the provision of accurate control of ignition timing at high and low load regimes. The usage of hydrogen as a fuel or hydrogen-containing fuel blends makes it possible to improve partly the HCCI performance. Today, synthesis gas (syngas), composed mostly of hydrogen and carbon monoxide, is considered as a very promising alternative fuel both for automobile and for gas turbine engines as well as for energetic machines [8e14]. Syngas possesses rather high mass heating value (for example, for syngas with the composition: H2/CO ¼ 1/ 1, it achieves 60 kJ/g, whereas for methane this magnitude is notably smaller, only of 50 kJ/g), and increasing flame speed compared to that for methane. Therefore, syngas combustion can be more stable near the fuelelean flammability limit compared to methane burning, and, thus, it becomes possible to reduce the concentration of NO, CO and CO2 in the combustion exhaust at low load. That is why some researchers have analyzed the characteristics of IC engines operating on syngas itself [10,11,15e19] or using syngas as an additive to traditional hydrocarbon fuels [19e22]. However, the principal disadvantages remain being unresolved even in this case. One of the ways to resolve the problems, arising upon HCCI combustion, is the addition in a small amount of chemical species promoting the combustion. Among such species, some researchers consider the nitric oxide NO [4,23,24]. Contino et al. [23] performed experiments in the HCCI engine operating on iso-octane/air mixtures and revealed a significant impact of NO addition on the combustion timing. The ignition delay decreases by 15 crank angle degrees for the case with 500 ppmv of NO compared to the case of NO absence. The same tendency in HCCI engine working on i-octane was also detected by Masurier et al. [24]. The less pronounced effect of NO addition was detected during the combustion of fuels containing n-heptane. Dubreuil et al. [4] observed that the maximum decrease of ignition delay with increasing initial NO concentration in the HCCI engine, operating on pure nheptane and two surrogate fuels containing n-heptane, was only 2 crank angle degrees for 100 ppmv of NO, and at higher NO concentration, no effect was noticeable. It is possibly caused by the impact of the cool flame in n-heptane/air mixtures. The numerical studies demonstrated that the promoting effect of NO was caused by the formation of highly reactive hydroxyl radical OH due to interaction of NO with hydroperoxyl radicals HO2 [25,26]. Therefore, the direct production of highly reactive species seems to be more promising way for the combustion enhancement. This may be such species as atomic oxygen, ozone or excited oxygen molecules and others produced in the initial charge via the activation of oxygen or air in the intake of cylinder by specially arranged electric discharge or by resonance laser radiation [3,24,27e32]. Tachibana et al. [27] studied experimentally the effect of ozone addition (up to 1000 ppm) to the intake charge on the combustion of three different fuels in the diesel engine. They revealed that the ozone addition shortened the ignition delay, somewhat increased the fuel cetane number and lowered the compression ratio of ignition limit. Moreover, under the fixed operation condition, it was

found the decrease of CO, unburned hydrocarbons and soot amount in the exhaust and small increase of NOx. Yamada et al. [3] showed experimentally that addition of ozone even in a small amount (0.015%) to dimethyl ester advanced the ignition timing notably. Simulation, performed in that work and based on the extended reaction mechanism taking into account ozone chemistry, allowed the authors to explain this effect. They concluded that the acceleration is caused by an increase of heat release in the cool flame. Nishida and Tachibana [28] observed experimentally that ignition timing in HCCI engine operating on natural gas could be controlled by changing the ozone concentration in the intake gas. On the base of the numerical analysis they concluded that the ozone addition enhanced the generation of OH via H2O2 molecules and that the influence of ozone addition is the same as the effect of O radical injection. Comprehensive analysis of the influence of trace amount of ozone, produced by electric discharge, on the ignition phase for n-heptane- and i-octane-fueled HCCI engine was done in Refs. [29] and [24], respectively. It was shown experimentally that low ozone concentrations (<50 ppm) have a considerable impact on the phasing of the cool and main flames in the engine. On the basis of numerical simulation the authors of these works showed that the main process responsible for the acceleration of ignition was the reaction of primary hydrocarbon (n-C7H16 or i-C8H18) with atomic oxygen produced via ozone decomposition. The effect of ozone addition on the combustion of six different PRF in a HCCI engine was studied in Ref. [30]. Experiments confirmed that injection of ozone could improve the combustion of each fuel and moved forward their phasing. At the identical concentration of O3, the effect is higher for fuels with the greater octane number. It was also shown that low concentrations of ozone (<20 ppm) in the intake charge yield a strong combustion advance. At higher ozone fraction, the combustion phasing continues to move forward but only moderately. It should be emphasized that kinetic analysis of the mechanisms responsible for the acceleration of ignition in the H2eair and CH4eair mixtures under ozone-seeded conditions was conducted in Refs. [33] and [34], respectively. Analysis of the possibility of HCCI combustion enhancement and reduction of NO and CO emissions for methanefueled HCCI engine upon the excitation of oxygen molecules to the singlet delta (a1Dg) electronic state was carried out by Starik et al. [31,32]. On the basis of numerical simulation with the usage of detailed reaction mechanism, involving the processes with O2(a1Dg) and O2(b1Sþ g ) molecules, it was shown that the presence of singlet delta oxygen (SDO) in the mixture could significantly intensify the ignition and combustion in HCCI engine. Moreover, in this case, it makes possible to increase the output energy and decrease the concentrations of NO and CO in the combustion exhaust. This paper does address the comparative analysis of the effect of ozone and SDO addition on the energetic and emission characteristics of HCCI engine operating on syngaseair mixture. The other goal of this paper is to highlight the kinetic mechanism responsible for the combustion enhancement in HCCI engine operating on syngas under ozone and SDOseeded conditions.

Please cite this article in press as: Starik AM, et al., Combustion improvement in HCCI engine operating on synthesis gas via addition of ozone or excited oxygen molecules to the charge: Modeling study, International Journal of Hydrogen Energy (2017), http://dx.doi.org/ 10.1016/j.ijhydene.2017.01.179

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Methodology and kinetic mechanism

Table 2 e Relative volume fractions of O2, O3 and O2(a1Dg) in total oxygen.

Analysis was carried out for the parameters and conditions typical for HCCI engine. These parameters are listed in Table 1. Zero-dimensional (0D) single-zone thermochemical model [35] was used for modeling. Such a model assumes the spatial uniformity of temperature, pressure and species concentrations inside the cylinder and cannot describe accurately some features of the real combustion during compression stroke (for example, the maximal values of pressure and temperature in the cylinder) and gives some inaccuracies in the predictions of NO and CO concentrations in the HCCI engine exhaust [19]. Nevertheless, such a model makes it possible to predict properly the energy released during HCCI combustion [19], because the ignition and combustion in HCCI is determined by the mixture parameters in the central part of cylinder volume. Moreover, this model is used by many researchers because it is very suitable for the analysis of chemical process development in mixtures with active species and different additives or in mixture blends, when the usage of detailed reaction mechanisms is necessary (see, for example, [36,37]). The effect of SDO and ozone abundance in the intake charge on the HCCI combustion was simulated by specifying a fraction of ground state oxygen O2(X3S g ) converted to excited O2(a1Dg) molecules. The amount of O2 converted to O2(a1Dg), was supposed to be equal to 0.1, 0.5 and 1%. The effect of SDO presence in the charge on the HCCI performance was compared with that of O3 availability when the identical energy is spent on the formation of these species. As is known, ozone forms during following processes

Specific input energy, eV/(O2 molecule)

O2 þ M ¼ 2O þ M and

0 9.8  104 4.9  103 9.8  103

O2(a1Dg) production

O3 production

Mole fraction O2

O2(a1Dg)

O2

O3

1 0.999 0.995 0.99

0.001 0.005 0.01

1 0.99942 0.9971 0.9942

e 0.00038 0.0019 0.0038

The detailed reaction mechanism, applied for the simulation, should include the physical and chemical processes with electronically excited species O2(a1Dg) and O2(b1Sþ g ) as well as with O3 molecules. Such a mechanism, developed earlier by Sharipov and Starik [38], was used as a basic one for the simulation of ignition and combustion of syngaseair mixture in HCCI engine. Note that this mechanism allows us to describe properly the experimental data on the ignition delay and flame speed in the syngaseair mixture [38,39]. In the present work, this mechanism was updated toward accounting for the novel data of ab initio calculations of the potential energy surface for the H þ O2(a1Dg) system and rate constants for the reactive H þ O2(a1Dg) ¼ OH þ O(3P)

(R1)

and quenching H þ O2(a1Dg) ¼ H þ O2(X3S g)

(R2)

channels, obtained in Ref. [40]. In line with that study, the temperature-dependent rate constants for the channels R1 and R2 can be expressed as following

O2 þ O þ M ¼ O3 þ M.

kR1(T) ¼ 1.17$107$T1.56$exp(630/T) þ 3.85$1010$T$exp(2480/T),

The former dissociation reaction requires considerably higher energy (5.1 eV/(O2 molecule)) than that for the excitation of O2 to the a1Dg state (0.98 eV/(O2 molecule)), for example, by the electron impact. Therefore, the smaller amount of O2 is converted into O3 at the same value of consumed energy. The corresponding relative volume fractions of O2, O3 and O2(a1Dg), for the cases under study, are given in Table 2. One can see that, at identical input energy, the amount of O3 molecules is approximately by a factor of 2.6 smaller than that of O2(a1Dg) ones.

kR2(T)¼ 5.36$106$T1.4$exp(810/T)þ 5.29$1010$T 0.68$exp(2825/T).

Table 1 e Engine parameters and operating conditions. Fuel Basic equivalence ratio, f Engine speed, rpm Compression ratio Cylinder volume, cm3 Cylinder clearance volume, cm3 Angle of intake valve closing, degree Angle of exhaust valve opening, degree P0, atm a

TDC denotes the Top Dead Center.

CO/H2 ¼ 1/1 0.5 1000 16.5 1575.87 103.3 142 before TDCa 115 after TDCa 1.065

It should be emphasized that H2 oxidation sub-mechanism taking into account these rate constants and involving the other processes with O2(a1Dg) and O2(b1Sþ g ) molecules as well as with O3 molecules allowed us to describe the experimental data [41] on shortening the ignition delay length in the H2eO2 mixture in a flow reactor upon the excitation of O2 molecule to the a1Dg state by low pressure glow discharge (see, for example [42]). This updated mechanism was supplemented by the submechanism of N-containing species formation including the reactions with O2(a1Dg) and O2(b1Sþ g ) molecules developed in Ref. [31].

Results and discussion As is known, combustion in HCCI engine is determined, generally, by chemical kinetics and, therefore, the ignition time is very sensitive to the charge temperature. In this work, the intake temperature of the charge was chosen by such a way to provide the ignition at the crank angle bign~10 after the

Please cite this article in press as: Starik AM, et al., Combustion improvement in HCCI engine operating on synthesis gas via addition of ozone or excited oxygen molecules to the charge: Modeling study, International Journal of Hydrogen Energy (2017), http://dx.doi.org/ 10.1016/j.ijhydene.2017.01.179

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top dead center (TDC) (this value of b corresponds to the maximal values of temperature and pressure achieved during the HCCI combustion at parameters under study). For the pure syngaseair mixture with the composition of syngas CO/ H2 ¼ 1/1 and fuel-to-air equivalence ratio f ¼ 0.5, such intake temperature of the charge turned out equal to Tin ¼ 386.3 K. The presence of highly reactive species SDO or O3 in the mixture intensifies the chain mechanism, and the ignition occurs earlier compared to the basic case. Even at 0.038% O3 content in oxygen, the mixture ignites at the crank angle of 1.5 before TDC. This is clearly seen from the temperature and pressure traces shown in Fig. 1 for different values of SDO and O3 fractions in oxygen. Fig. 2 depicts the variation of species mole fractions gi versus crank angle b during combustion of syngaseair mixture in the basic case upon the absence of adding SDO and ozone in the intake charge, and upon the presence of 1% SDO or equivalent amount of ozone molecules (at the same input energy) (see Table 2). One can see that the abundance of O2(a1Dg) and O3 molecules leads to more rapid initiation of chain mechanism of syngas oxidation and, as a result, to faster ignition of the mixture compared to the basic case. It is also seen that chain process in the H2eCOeair mixture upon the presence of SDO or ozone molecules develops differently. Analysis of reaction rates showed that, in the case of SDO abundance in the mixture, the chain process of syngas oxidation is initiated by following reactions

H2 þ OH ¼ H2O þ H

(R7)

also contributes to the chain mechanism development. Precisely these processes are responsible for the intensification of ignition in HCCI engine in this case. Analysis also showed that SDO molecules are partly quenched during the time interval from the intake of the charge (b ¼ 142 ) up to the time instant corresponding to b ¼ 60 , when the temperature achieves the value T~700 K, and the rates of chainbranching reactions become greater than that of chain termination reaction. Due to quenching of SDO in the course of reaction O2(a1Dg) þ M ¼ O2(X3S g ) þ M, where M is the any colliding particle, the temperature increases approximately by 15 K compared to the basic case. In order to avoid quenching of O2(a1Dg) molecules, it is needed to produce SDO not in the intake charge, but at some optimal value of crank angle before the ignition event [32]. This results in greater rates of chain reactions and stronger influence on ignition and combustion phasing in HCCI engine. In the case of the abundance of ozone in the intake charge, one can observe two stages of ignition phasing. At the first stage, due to decomposition of ozone O3 þ M ¼ O2 þ O þ M,

H2 þ O2(a1Dg) ¼ H þ HO2

(R3)

and CO þ O2(a1Dg) ¼ CO2 þ O.

(R4) H þ O2 ¼ OH þ O.

Then, the reactions of chain-branching H þ O2(a1Dg) ¼ OH þ O

(R5)

H2 þ O ¼ OH þ H

(R6)

come into play. The chain propagation reaction

atomic oxygen arises at the beginning of compression stroke (see Fig. 2c). It initiates chain process of syngas oxidation via the Reaction R6 and reaction (R8)

Atomic oxygen also reacts with CO with the formation of CO2 at the early stage. However, ozone eliminates practically completely up to the time instant corresponding to the crank angle b ¼ 35 (see Fig. 2c), and, due to ozone burning, the temperature increases by 20 K compared to the basic case. At such a value of crank angle, the temperature in the cylinder

Fig. 1 e Variation of temperature T and pressure P in HCCI engine operating on syngaseair mixture (CO/H2 ¼ 1/1) at Tin ¼ 386.3 K and f ¼ 0.5 in the case of the absence of active molecules in the intake charge (curve 1) and in the case of the presence of 0.1, 0.5 and 1% O2(a1Dg) in total oxygen (solid curves 2e4) or equivalent amount of O3 molecules (dashed curves 2e4). Please cite this article in press as: Starik AM, et al., Combustion improvement in HCCI engine operating on synthesis gas via addition of ozone or excited oxygen molecules to the charge: Modeling study, International Journal of Hydrogen Energy (2017), http://dx.doi.org/ 10.1016/j.ijhydene.2017.01.179

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Fig. 2 e Variation of temperature T and species mole fractions gi in HCCI engine operating on syngaseair mixture (CO/ H2 ¼ 1/1) at Tin ¼ 386.3 K and f ¼ 0.5 vs. crank angle b in the case of the absence of active molecules in the intake charge (a) and upon the presence of 1% SDO in total oxygen (b) or equivalent amount of O3 molecules (c).

(T ¼ 750 K) is not sufficient for ignition, because the rate of chain termination reaction

maximal value of pressure Pmax. The growth of Pmax results in Z V1 PdV, where V0 and V1 the increase of output energy Ec (Ec ¼

H þ O2 þ M ¼ HO2 þ M

are the values of cylinder volume available for combustion at the values of crank angle b ¼ 142 and 115 , respectively) released during combustion and the power W (W ¼ 2Ec/t, where t is the time of the revolution of crank arm) by 7 and 14% for the cases of 1% SDO and equivalent O3 content (0.38%) in oxygen, respectively. However, the decrease in the intake temperature implies the higher value of fuel mass m put into the cylinder compared to the basic case. Therefore, the value of specific power Wm ¼ W/m increases by 1e2% only for these cases. The decrease of maximal temperature Tmax in the cylinder upon the presence of SDO or O3 molecules in the charge results in reducing NO mole fraction in combustion products, because the formation of NO, for the cases under study, is determined mostly by thermal mechanism. It should be noted that zero-dimensional single-zone thermochemical model predicts somewhat higher temperature value in the cylinder compared to that obtained during 2D CFD modeling, because this model does not take into account the heat loss through the cylinder wall as well as nonuniformity of thermodynamic parameters inside the cylinder. In reality, these effects ensure slower temporal evolution of temperature and pressure inside the cylinder and, as a result, smaller NO concentration in the combustion exhaust (see, for example, [19]). However, the

V0

(R9)

is higher, in this case, than the rate of O atom production (Reaction R8). The rise in the temperature due to further compression of the charge leads again to the growth of the concentrations of active atoms O, H and radical OH and, as a result, to the ignition of the mixture. The ignition before TDC can result in the knock in the engine cylinder and even the engine damage. In order to ensure the ignition at needed value of crank angle (bign~10 ) in the case of the abundance of active species, the intake temperature of the charge should be reduced. Presented in Table 3 are the values of Tin required for the ignition at bign~10 at different content of SDO and O3 molecules in total oxygen. One can see that the higher is the fraction of active species in oxygen, the smaller temperature of intake charge should be taken. The production of O3 must be accompanied by larger decrease of Tin, than that upon the generation of SDO molecules in the intake charge. In the case of 0.38% O3 content, it is necessary to decrease Tin by 42 K, whereas, in the case of 1% SDO mole fraction in total oxygen e by 22 K. The decrease of Tin leads to diminishing the maximal temperature Tmax in the cylinder during the compressioneexpansion stroke as well as to the growth of the

Please cite this article in press as: Starik AM, et al., Combustion improvement in HCCI engine operating on synthesis gas via addition of ozone or excited oxygen molecules to the charge: Modeling study, International Journal of Hydrogen Energy (2017), http://dx.doi.org/ 10.1016/j.ijhydene.2017.01.179

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Table 3 e Intake temperature of the charge Tin, at which the ignition of syngas (CO/H2 ¼ 1/1) in air (f ¼ 0.5) with abundance of SDO or O3 molecules occurs at bign ¼ 10 , as well as input energy Es, maximal temperature Tmax and pressure Pmax, energy Ec released during combustion, power W, specific power Wm and relative mole fractions of NO and CO gNO =g0NO and gCO =g0CO (in respect to the basic case) in the HCCI combustion exhaust achieved for the cases considered. Active molecule e O2(a1Dg)

O3

Active molecule mole fraction in total oxygen

Es, J

Tin, K

Tmax, K

Pmax, atm

Ec, J

W, kW

Wm, kW/g

gNO g0NO

gCO g0CO

0% 0.1% 0.5% 1% 0.038% 0.1% 0.38%

0 0.83 4.26 8.68 0.85 4.45 9.17

386.3 379 370.5 364 369.9 354.5 344.5

2560 2552 2546 2538 2537 2519 2504

80.7 81.5 83.2 84.4 82.2 86.2 87.6

1054 1077 1107 1131 1106 1167 1206

35.1 35.9 36.9 37.7 36.9 38.9 40.2

271 272 273 274 273 276 277

1 0.94 0.89 0.86 0.84 0.76 0.68

1 0.94 0.89 0.85 0.88 0.77 0.71

relative variation of NO emission index due to addition of SDO or ozone in the charge can be predicted by this model with reasonable accuracy. The computations also showed that abundance of SDO or O3 molecules in the intake charge made it possible to reduce CO emission. The main reason of the decrease of CO production during HCCI combustion in these cases is the acceleration of CO oxidation due to formation greater amount of highly reactive OH radicals caused by the reactions with O2(a1Dg) and ozone molecules. The results of the calculations of energetic characteristics (energy released during combustion Ec, engine power W and specific power Wm) and normalized mole fractions of NO and CO in combustion exhaust (in respect with the basic case) gNO =g0NO and gCO =g0CO , respectively, for the cases considered, are summarized in Table 3. The energy put into the gas Es, needed for the production of the certain amount of SDO and O3 molecules, is also presented there. When analyzing the data listed in Table 3, one can conclude that the production of active molecules O2(a1Dg) or O3 in the intake charge even in a small amounts allows one to organize the combustion in HCCI engine with smaller emission of NO and CO and somewhat greater specific power compared to the basic case. So, the reduction in the NO and CO emissions at 1% of SDO content in total oxygen can mount to 15% and upon equivalent concentration of ozone (0.38%) achieves even 30%. It is remarkable that the energy Es~9 J, put into the gas in this case, is by an order of magnitude smaller (by a factor of 8.6

Fig. 4 e PV diagram for the HCCI engine operating on syngaseair mixture (CO/H2 ¼ 1/1) at Tin ¼ 386.3 K and f ¼ 0.44, 0.5 and 0.7.

for the presence of SDO and by 16.6 times for the equivalent amount of O3) than the advance in the energy DEc released during combustion (DEc ¼ EcE0c , where E0c is the value of Ec for the basic case). Consider now the combustion of syngas in HCCI engine at the basic intake charge temperature Tin ¼ 386.3 K and different f values. The traces of temperature and pressure for f ¼ 0.43,

Fig. 3 e Variation of temperature T and pressure P in the cylinder of HCCI engine operating on syngaseair mixture (CO/ H2 ¼ 1/1) as a function of crank angle b at Tin ¼ 386.3 K and f ¼ 0.43, 0.44, 0.5, 0.6 and 0.7. Please cite this article in press as: Starik AM, et al., Combustion improvement in HCCI engine operating on synthesis gas via addition of ozone or excited oxygen molecules to the charge: Modeling study, International Journal of Hydrogen Energy (2017), http://dx.doi.org/ 10.1016/j.ijhydene.2017.01.179

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Fig. 5 e Variation of temperature T and pressure P in the cylinder of the HCCI engine operating on syngaseair mixture (CO/ H2 ¼ 1/1) as a function of crank angle b at f ¼ 0.44 without addition of active species (thick solid curve) and at f ¼ 0.4, 0.3 and 0.2 with 0.1% of SDO content (thin solid curves) or with equivalent amount of O3 molecules (dashed curves). Tin ¼ 386.3 K.

Fig. 6 e Engine power W as a function of f value for the HCCI engine operating on syngaseair mixture (CO/H2 ¼ 1/ 1) at Tin ¼ 386.3 K in the case of the absence of active molecules in the intake charge (curve 1) and upon the presence of 0.1% SDO (curve 2) or equivalent amount of O3 molecules in total oxygen (curve 3).

0.44, 0.5, 0.6 and 0.7 are depicted in Fig. 3. One can see that the decrease of f value decelerates the ignition (for the mixture with f ¼ 0.44, ignition occurs at bign~18 , whereas, at f ¼ 0.5, the mixture ignites at bign~10 ). The smaller is the f value, the smaller are the maximal values of temperature and pressure as well as the temperature and pressure of combustion products. As a result, the energy, released during the compressioneexpansion stroke, decreases. This is clearly seen from the PV diagram shown in Fig. 4. It should be emphasized that upon the absence of active species in the intake charge, the ignition occurs at f  0.44 for conditions considered (see Fig. 3). The presence of 0.1% SDO or equivalent amount of O3 molecules provides the ignition at f ¼ 0.2 and 0.18, respectively, i.e. the generation of even small amounts of O2(a1Dg) or O3 molecules shifts the ignition threshold in HCCI engine toward the fueleleaner mixture. This is illustrated in Fig. 5, which depicts the temperature and pressure traces at different f values when 0.1% of SDO or equivalent amount of O3 is present in the intake charge. The power, at very small f values (f~0.2), is approximately by a factor of 2 smaller than that at f ¼ 0.5, however, the engine continues to

Fig. 7 e Emission indices of CO and NO as a function of f value for HCCI engine operating on syngaseair mixture (CO/H2 ¼ 1/ 1) at Tin ¼ 386.3 K in the case of the absence of active molecules in the intake charge (curve 1) and upon the presence of 0.1% SDO (curve 2) or equivalent amount of O3 molecules in total oxygen (curve 3). Please cite this article in press as: Starik AM, et al., Combustion improvement in HCCI engine operating on synthesis gas via addition of ozone or excited oxygen molecules to the charge: Modeling study, International Journal of Hydrogen Energy (2017), http://dx.doi.org/ 10.1016/j.ijhydene.2017.01.179

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Fig. 8 e Normalized mass of NO mNO =m0NO (in respect with the total mass of NO, m0NO, formed in the case of the absence of active molecules in the intake charge at f ¼ 0.5) produced via thermal and N2O mechanisms for f ¼ 0.5, 0.33 and 0.25 in HCCI engine operating on syngaseair mixture (CO/H2 ¼ 1/1) at Tin ¼ 386.3 K in the case of the absence of active molecules in the intake charge and upon the presence of 0.1% SDO or equivalent amount of O3 molecules in total oxygen.

operate. This is clearly seen from the dependence W(f) shown in Fig. 6. Thus, one can conclude that the production of active molecules, such as O2(a1Dg) or O3, in the intake charge expands the diapason of the stable operation of HCCI engine and makes it possible to control the ignition phasing at low load regime. Upon the operation of HCCI engine on the fueleleaner mixture, the smaller CO emission takes place. This is seen from the plots shown in Fig. 7. So, in the case of 0.038% O3 content in oxygen, when the mixture ignites at f ¼ 0.2, the decrease in the emission index EICO achieves a factor of 3 compared to the basic case at f ¼ 0.5. Herein and hereafter the emission index of M-th component of the mixture is defined as the amount of M-th species in grams produced upon burning of 1 kg of the fuel. Note that the sharp growth of EICO magnitude at some boundary value of f: fb ¼ 0.44 for the basic case, fb ¼ 0.2 and 0.18 for the cases with addition of 0.1% SDO and 0.038% O3 to the charge is caused by the absence of ignition at f
because O2 is spent practically completely on the oxidation of the fuel. The possibility of the decrease of f value from 0.5 to ~0.2 due to the presence of SDO or ozone in the intake charge allows one to reduce substantially the NO concentration in the combustion products. At f ¼ 0.5, the maximal temperature is rather high, and NO forms mainly due to the thermal mechanism. N2O-mechanism gives only 5e10% of the total amount of NO in this case. For the fueleleaner mixture with f < 0.5, the contribution of N2O-mechanism becomes more pronounced. So, at f ¼ 0.25, thermal and N2O mechanisms contribute identically to the NO formation. This is illustrated by the diagram shown in Fig. 8. Thus, the production of small amount of SDO or O3 molecules (only 0.1% of SDO or 0.038% of O3 in total oxygen) makes it possible to provide small emissions of NO and CO at low load regime of HCCI engine operating on syngaseair mixture.

Conclusions The comparative analysis of the effect of the addition of SDO or ozone on the energetic and emission characteristics of HCCI engine, operating on syngaseair mixture, was conducted. It was shown that the abundance of SDO or O3 molecules in the charge essentially accelerates the ignition, which can occur before TDC. To avoid earlier ignition, the temperature of the intake charge should be diminished. At identical ignition moment, the presence of 1% of SDO or equivalent amount of O3 (0.38%) in total oxygen can increase the power value by 7e14% compared to the basic case with higher intake temperature. However, due to the greater fuel mass put into cylinder at lower temperature, the specific power value Wm increases only by 1e2%. Nevertheless, such a way of arrangement of combustion in HCCI engine is very promising even for nominal regime, because it makes possible to reduce simultaneously NO and CO emissions (up to 15% for 1% of SDO and up to 30% for equivalent amount of O3 production). It is remarkable that the energy, put into the gas to produce certain amount of SDO or ozone, is approximately by an order of magnitude smaller than the advance in the energy released during HCCI combustion compared to the basic case. More promising effect was predicted when SDO or O3 molecules were used for supporting the combustion of extremely fuelelean mixture with f ¼ 0.2 (low load regime). In this case, the production of very small amount of SDO or ozone (0.1% SDO of total oxygen or equivalent amount of O3 (0.038%)) ensures the stable ignition of such a fuelelean mixture in HCCI engine. Though the engine power, at such extremely small f value, is rather small, the engine continues to operate even at low load with small emissions of CO and NO. The production of O3 molecules in the intake charge is more effective compared to the production of equivalent amount of SDO in the viewpoint of the control of ignition phasing and enhancement of combustion in HCCI engine operating on syngaseair mixture. It is worth noting that the efficiency of the production of SDO in the HCCI combustion enhancement might be higher in the case of the excitation of O2 molecules to the a1Dg state directly in the cylinder over the piston at some optimal value of crank angle.

Please cite this article in press as: Starik AM, et al., Combustion improvement in HCCI engine operating on synthesis gas via addition of ozone or excited oxygen molecules to the charge: Modeling study, International Journal of Hydrogen Energy (2017), http://dx.doi.org/ 10.1016/j.ijhydene.2017.01.179

i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e1 0

Acknowledgements [15]

This work was supported by Russian Foundation for Basic Research (project no. 14-01-00464-a and 14-08-00743-a) 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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Appendix A. Supplementary data

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Supplementary data related to this article can be found at http://dx.doi.org/10.1016/j.ijhydene.2017.01.179. [19]

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