Article pubs.acs.org/EF
Reduced Polycyclic Aromatic Hydrocarbon Formation Chemical Kinetic Model of Diesel Surrogate Fuel for Homogeneous Charge Compression Ignition Combustion Feng Wang, Zhaolei Zheng,* and Zuwei He Key Laboratory of Low-Grade Energy Utilization Technologies and Systems, Ministry of Education, Chongqing University, Chongqing 400030, People’s Republic of China S Supporting Information *
ABSTRACT: A reduced mechanism, which could couple with a multi-dimensional computational fluid dynamics code, was developed for the chemical kinetics of diesel surrogate fuel oxidation in modeling polycyclic aromatic hydrocarbon (PAH) formation under homogeneous charge compression ignition (HCCI) diesel engine conditions. The complete kinetic mechanism, which comprised 697 reactions and 153 species, was reduced to a minor mechanism that included only 141 reactions and 75 species using the sensitivity and reaction path analyses. Validation of the present mechanism was also performed with experiments from the shock tube available in the literature, and good agreement between modeling results of the detailed mechanism in the shock tube and HCCI engine was obtained. The results showed that this reduced mechanism gave reliable performance for HCCI combustion predictions. Numerical results also displayed that those PAH concentrations decreased with the increase of the inlet air temperature and equivalence ratio.
1. INTRODUCTION The increase of the environmental concern and the more stringent regulations about pollutant emissions from internal combustion engines make the search of alternatives for current automotive engine combustion processes necessary. One of the technologies that is receiving attention is the homogeneous charge compression ignition (HCCI) based on the combination of traditional spark ignition (SI) and compression ignition (CI) processes, providing very low NOx and particulate matter (PM) emissions while keeping a good fuel economy.1 However, with the development of the studies on HCCI and a deeper understanding of HCCI combustion, researchers realize that diesel HCCI combustion, in a generalized concept, does not mean absolute uniform mixture combustion in the cylinder, including a series of new combustion modes, such as stratified combustion, low-temperature combustion, etc., which exist because of the stratified concentration in the cylinder gas mixture. This new kind of combustion mode is mainly about premixed combustion. Largely because of the avoidance of the diffusion combustion stage of traditional diesel combustion, the PM mass concentration is extremely low. Therefore, the chemical kinetic model of this kind of combustion mode is different from the traditional diesel engine and gasoline engine. The need to control the emission of combustion products while also promoting more efficient use of fossil energy resources requires the development of cleaner and more economic combustion equipment, which, in turn, requires a better physical and chemical understanding of combustion processes. As we all know, HCCI combustion is dominated by fuel chemical kinetics.2 Thus, a detailed model that properly describes fuel oxidation chemistry is essential to model diesel HCCI combustion. However, commercial diesel fuel consists of a complex mixture of hundreds of medium−high-molecular© 2012 American Chemical Society
weight hydrocarbons. It is not feasible to consider the oxidation chemistry of all of the compounds for modeling targets. n-Heptane is usually chosen as the substitute of diesel fuel because its approximation in the cetane number (CN) and its oxidation chemistry are very well-known. Detailed n-heptane mechanisms with hundreds of species and thousands of reactions have been developed,3 but the calculation cost of these detailed kinetic mechanisms is too large to be coupled with three-dimensional (3D) computational fluid dynamics (CFD); therefore, some reduced mechanisms4,5 were developed on the basis of detailed mechanisms. The skeletal nheptane mechanism developed by Reitz and co-workers4 with 29 species and 52 reactions has been widely used in diesel combustion modeling. Reactions of acetylene were then added to this reduced mechanism for soot formation modeling. Xi and Zhong6 and Zhao et al.7 constructed a kinetic model for polycyclic aromatic hydrocarbon (PAH) formation in diesel combustion. They added the main PAH formation reactions that were presented by Wang and Frenklach8 to n-heptane mechanism. In their model, n-heptane was used as the diesel surrogate fuel. However, the single component does not reproduce well the ignition characteristics of the actual diesel fuel. Commercial diesel fuels contain 30−35% aromatics, except paraffin,9 because of the very high aromatic content of diesel, which causes a significant decrease in the fuel reactivity and has effects on PAH formation. Hence, some authors10,11 suggest the consideration of the substitute that includes the linear paraffin content and aromatic compound mixture representing diesel fuel. Ramirez et al.12 investigated the kinetics of oxidation Received: December 9, 2011 Revised: February 7, 2012 Published: February 8, 2012 1612
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of a commercial diesel fuel and a diesel surrogate fuel (70% ndecane/30% 1-methylnaphthalene in moles) in a jet-stirred reactor at elevated pressures. The experimental results showed that the two fuels oxidize very similarly. In addition, the kinetic modeling showed reasonable agreement between their experimental results and computations over the entire range of conditions. Kook and Pickett13,14 investigated a surrogate of 77% n-dodecane and 23% m-xylene and compared it to a conventional jet fuel in a high-pressure and high-temperature combustion vessel used to simulate conditions in a diesel engine. Golovitchev15 used the n-heptane and toluene mixture as the diesel surrogate fuel and proposed a kinetic mechanism with 75 species and 329 reactions to model combustion and soot formation in diesel engines. However, the PAH chemical kinetics model contained not more than two fused rings, which was not suitable to simulate the soot formation. On the basis of previous studies, we constructed a complete kinetic model for PAH formation in diesel HCCI combustion, which comprised 697 elementary reactions and 153 different chemical species. Nevertheless, this detailed kinetic mechanism was too complex to couple with a multi-dimensional CFD code. In the current paper, a new reduced PAH formation chemical kinetic model of diesel surrogate fuel has been developed on the basis of the detailed mechanism by sensitivity analysis and reaction path analysis in HCCI combustion. Then, the reduced model has been validated against the experimental data of the shock tube and simulated results from a detailed model, to assess its capability to predict HCCI combustion.
mechanism should be simplified. In the present study, the PAH formation chemical kinetic model of n-heptane/toluene diesel surrogate fuel was reduced from 153 species and 697 reactions to 75 species and 141 reactions using sensitivity and reaction path analyses under HCCI combustion conditions. 3.1. Sensitivity Analysis. Sensitivity analysis represents the ensemble response to small disturbances by calculating the ensemble property parameter (e.g., species concentration, temperature, or reaction rate) variation caused by small changes in the value of a reaction rate constant. That variation is the so-called sensitivity, which can be valued via sensitivity coefficients resolved through a series of differential equations. The magnitude of each elemental reaction sensitivity coefficient value represents its relative significance in the ensemble. When minor reactions are eliminated, a simplified chemical mechanism will be obtained.21 The normalized matrix of the sensitivity coefficients is defined as ∼
S =
k j ∂ci ∂ ln ci = ci ∂k j ∂ ln k j
(1)
where S̃ is the matrix of normalized sensitivity coefficients, kj is the jth reaction rate constant, ci is the ith species concentration, and ∂ci/∂kj is the sensitivity coefficient of the ith species concentration to the jth reaction rate constant. If the sensitivity coefficient of the temperature to elemental reaction rate constants was to be considered, eq 1 can be expressed as ∼
T =
2. DETAILED KINETIC MODEL According to the method by Tao et al.,16 the entire combustion mechanism contains two parts: the chemical kinetic model of diesel surrogate fuel and the formation reactions of PAHs (up to five aromatic rings). In this kinetic model, n-heptane and toluene are used as diesel surrogate fuel composition, which are representative of the alkane and aromatic contents of the diesel fuel, respectively. In the first part, the diesel surrogate fuel kinetic mechanism is built up on a stepwise fashion, using as a starting point the n-heptane skeletal mechanism proposed by Peters et al.,17 which describes both the high- and low-temperature chemistry and has been shown to reproduce ignition delay times at various pressures and temperatures reasonably well. Then, the reactions needed to describe the oxidation of toluene were selected from those proposed by Andrae et al.18 In addition, cross reactions based on the work by Andrae et al.18,19 between the oxidation of alkane and toluene are also of importance, which are considered in the detailed model. In the second part, several key reactions related to PAH formation, which were developed by Wang and Frenklach,8 has been chosen. Besides, main reactions, including the reaction mechanism of INDENE (C9H8), acenaphthylene (C12H8), BGHIF (C18H10), chrysene (C18H12), and BAPYR (C20H12), were added to the PAH formation and oxidation mechanism, which were abstracted from the detailed mechanism developed by Slavinskaya and Frank.20 The names and molecular formulas of PAHs are shown in the Supporting Information. The detailed mechanism comprised 153 species and 697 elemental reactions, which was validated as being able to predict quantitatively the results of experiments and could model the diesel surrogate fuel HCCI combustion process.
k j ∂T ∂ ln T = T ∂k j ∂ ln k j
(2)
where T̃ is the matrix of the normalized temperature sensitivity coefficients and T is the system temperature. Because the temperature is very sensitive to ignition of the HCCI engine, we keep the important reactions using the method of the temperature sensitivity analysis. The HCCI engine used for the calculation is a single-cylinder diesel engine, and the main engine specifications are shown in Table 1. The Table 1. HCCI Engine Specifications HCCI engine bore stroke connecting rod length compression ratio clearance volume engine speed inlet valve closing (IVC) exhaust valve opening (EVO)
115 mm 115 mm 210 mm 17 74.656 cm3 1400 revolutions/min 135 BTDC 125 ATDC
four different operating conditions (OP1−OP4) of the HCCI engine in the simulation are given in Table 2. The mole fraction of n-heptane/toluene keep 7:3 in the new model, because the 70:30% mixture of n-heptane/toluene can sufficiently represent the cetane number as well as the other properties of real fuel.22−24 Figure 1 shows the results of the temperature sensitivity analysis. Through the four cases of the temperature sensitivity analysis, we keep 19 reactions whose reaction rate has a significant effect on the system temperature changes. The corresponding element reactions are shown in Table 3.
3. REDUCED MODEL FORMATION While the detailed chemical kinetic model coupled with the calculation of a multi-dimensional turbulent CFD code, the computation burden would be too heavy under current computer capacities. Thus, a detailed chemical kinetic 1613
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Previous studies had shown that the sensitivity analysis method could find out most of the key reactions, but it could not construct a complete simplified dynamics model, which represented the combustion process of diesel surrogate fuel. Hence, sensitivity analysis was applied in combination with other methods. 3.2. Reaction Path Analysis. According to the main path of diesel surrogate fuel (n-heptane/toluene fuel) and PAH formation, the relevant element reactions were added. The main chemical reaction paths of n-heptane and toluene was derived from ref 25. Figures 2 and 3 present the chemical reaction paths of n-heptane and toluene, respectively. The PAH formation and oxidation mechanism was taken from Wang and Frenklach,8 and the PAH formation and oxidation reaction proceeds primarily through A1 → INDENE (C9H8) → A2 → A2R5 → A3 → A4 → C18H12 → BAPYR (C20H12). However, the critical reactions above indicated that some intermediate species were introduced into the reduced mechanism. A complete model should have not only generation paths but also consumption paths of these species to convert them to the ultimate products. Then, we kept the reactions that has a significant impact on generation and consumption of intermediate species using the reaction rate analysis. Using the method of sensitivity and reaction path analyses, a reduced mechanism was obtained that consisted of 75 species and 141 elementary reactions. These reactions and their rate parameters are listed in the Supporting Information.
Table 2. HCCI Operating Conditions OP1 OP2 OP3 OP4 OP5 OP6 OP7 OP8 OP9
intake temperature (K)
intake pressure (atm)
equivalence ratio
350 330, 350, 370, 390 350 350 350 370 350 350 330, 360
1 1 1, 1.5, 2 40, 60, 80 1 1 1 1.2 1
0.264, 0.4, 0.7 0.264 0.264 0.264 0.264 0.264 0.3 0.264 0.2−0.6
In this study, the necessary chemical species were selected on the basis of the following considerations: First, the reactants and products of the above 19 reactions were used as basic species. Second, fuel, oxidant, and the major production substances were prerequisites in the chemical reaction ensemble, which constructed the fundamental reaction pathways for diesel surrogate fuel oxidation. Third, PAHs were the objective species of concern. Finally, we obtained 34 necessary chemical species (C 7 H 16 , C 7 H 15‑1 , C 7 H 15‑2 , C 7 H 15 O 2 , C7H14O2H, OC7H13O2H, OC7H13O, O2, OH, C5H11, PC4H9, C 3 H 6 , C 2 H 4 , C 6 H 5 CH 3 , C 6 H 5 CH 2 , C 6 H 5 OH, C 6 H 5 O, C6H4CH3, OC6H4CH3, C3H5, C2H3, CH2O, CO, H, HO2, H2O, H2O2, CO2, A1, A2, A2R5, A3, A4, and BAPYR) using the above method. Then, the relevant element reactions were determined using the species sensitivity analysis in the four different operating conditions (Table 2)
Figure 1. Results of the temperature sensitivity analysis in four different cases. 1614
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Table 3. Temperature Sensitivity Analysis Results number
reaction
number
reaction
5 6 10 11 12 16 17 18 19 26
C7H16 + OH → C7H15‑1 + H2O C7H16 + OH → C7H15‑2 + H2O C7H15‑1 + O2 = C7H15O2 C7H15‑2 + O2 = C7H15O2 C7H15O2 → C7H14O2H OC7H13O2H → OC7H13O + OH OC7H13O2 → CH2O + C5H11 + CO C7H15‑1 → C5H11 + C2H4 C7H15‑2 → PC4H9 + C3H6 C6H5CH3 + OH = C6H5CH2 + H2O
43 49 50 60 67 94 101 108 109
C6H5OH + OH = C6H5O + H2O C6H5CH3 + OH = C6H4CH3 + H2O C6H4CH3 + O2 = OC6H4CH3 + O C3H6 + OH = C3H5 + H2O C2H4 + OH = C2H3 + H2O CO + OH = CO2 + H OH + HO2 = H2O + O2 H2O2 + M = OH + OH + M H2O2 + OH = H2O + HO2
mechanism is able to predict ignition delay times, model predictions have been compared to experimental results referred to shock tube investigations. All computed data of ignition delay times have been obtained by means of the closed homogeneous constant volume reactor model from the CHEMKIN 4.1 program package,26 under the assumptions of a homogeneous, constant volume and adiabatic conditions. Ignition delay times are defined as the times needed for the mixture to reach a temperature of 400 K above the initial temperature. All of the figures of ignition delay are shown in semi-log plot. The ignition delay times were simulated using the reduced and detailed mechanism compared to the experimental data. Figure 4 shows computed ignition delay times of the nheptane/air mixture at different equivalence ratios (ϕ = 0.5 and 1.0) and pressures (13.5 and 42 atm) compared to the shock tube experimental data by Ciezki and Adomeit.27 In this figure, it can be seen that the reduced model is sensitive to the change in temperature and captures the negative temperature coefficient (NTC) region as well as the detailed model. The agreement between experimental and computed values is quite a good trend. Figure 5a is the comparison of ignition delay times of the toluene/air mixture between the calculation and the shock tube experiments by Davidson et al.,28 at initial pressures of 50 atm
Figure 2. Major reaction branches of n-heptane oxidation.
4. MODEL VALIDATION 4.1. Comparison to Experimental Data of Ignition Delay Times in Shock Tubes. To show how the new reduced
Figure 3. Major reaction branches of toluene oxidation. 1615
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Figure 4. Comparison of experimental and calculated ignition delay times of the n-heptane/air mixture at different pressures and equivalence ratios.
Figure 5. Comparison of experimental and calculated ignition delay times of the toluene/air mixture at different pressures and equivalence ratios.
temperature and pressure curves obtained by the simplified mechanism are in acceptable agreement with those of the detailed mechanism. Therefore, this reduced mechanism can predict the temperature and pressure profiles that are essentially identical to those predicted by the detailed mechanism. Figure 7 shows the mole fraction comparison of concerned species, including CO2, H2O, H2O2, and OH, at two different operating conditions (OP6 and OP7 in Table 2). It can be observed that species mole fraction profiles obtained by the simplified mechanism are slightly smaller than those of the detailed mechanism. The reason for this is that every species has a lot of generation and consumption paths in the elementary reactions of the detailed mechanism. To achieve the purpose of simplification, we choose only one more important path. However, the trends of the calculation results are captured quite well by the reduced model. In conclusion, the detailed mechanism can be represented by this reduced mechanism in modeling the n-heptane/toluene mixture fuel for HCCI combustion chemistry with acceptable accuracy.
and stoichiometric ratio. Figure 5b gives the comparison of ignition delays between the calculated results and the experimental results measured by Pitz et al.10 in the shock tube for toluene at a pressure of 9 atm and equivalence ratios (ϕ = 0.5, 1.0, and 1.5). The reduced model does a relatively good job in predicting the ignition delay times and can also predict the difference in ignition delay times between ϕ = 0.5, 1.0, and 1.5. The ignition delay times decrease with an increasing equivalence ratio. To be noted, the ignition delay times predicted by the present reduced mechanism are almost the same as that by the detailed mechanism. 4.2. Comparison to Simulated Results of the Detailed Model in HCCI Engines. The reduced model has been validated against the simulated results from a detailed model, to assess its capability to predict HCCI combustion. A single zone model is used in the computation, assuming that the temperature, pressure, and species concentrations are uniform in the combustor. All computed data have been obtained by applying the closed internal combustion simulator module from the CHEMKIN 4.1 program package.26 The HCCI engine specifications are the same as in the above paragraphs. Figure 6 displays the comparison of the temperature and pressure curves in the cylinder at four different operating conditions (OP5−OP8 in Table 2). It can be seen that the 1616
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Figure 6. Comparisons of the in-cylinder temperature and in-cylinder pressure profiles.
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Figure 7. Comparisons of main species mole fraction profiles.
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Figure 8. Temporal variations of the temperature and equivalence ratio for PAH species during the HCCI combustion process.
5. RESULTS AND DISCUSSION
6. CONCLUSION A new reduced PAH formation chemical kinetic model of diesel surrogate fuel for HCCI combustion has been developed. The established model consists of 75 species and 141 reactions, which is a substantial reduction from the original model of 153 species and 697 reactions, using the sensitivity and reaction path analyses. In shock tube conditions, the computed ignition delay time with the new reduced mechanism was validated against the experimental data in the literature and computation results of the detailed mechanism with the n-heptane/toluene mixture as fuel at different conditions. Good agreements of ignition delay times were obtained. The performance of the reduced mechanism was tested under the HCCI combustion conditions. It was found that the predictions of the temperature, pressure profile, and product concentrations from the reduced model were largely indistinguishable from those of the detailed model over a fixed range of reaction conditions. The presented reduced mechanism can be applied to the combustion and emission simulation by coupling to the CFD model in internal combustion engines. In HCCI combustion conditions, PAH concentrations decrease with the increase of the inlet air temperature and equivalence ratio.
To investigate the effect of the inlet air temperature and equivalence ratio on PAH production quantitatively, the closed internal combustion simulator module from the CHEMKIN 4.1 program package26 is also used with the new reduced model under HCCI diesel engine conditions. The engine specifications and calculation conditions in the simulation are shown in Tables 1 and 2, respectively. The model predictions for the mole fraction of PAH as a function of the equivalence ratio at two inlet air temperature conditions, i.e., 330 and 360 K (OP9), is shown in Figure 8. The simulations are made separately for various equivalence ratios, e.g., from 0.2 to 0.6. From Figure 8, some observations can be made according to model predictions. First, with the change of the inlet air temperature from 330 to 360 K at a fixed equivalence ratio, the PAH quantities decrease, which is because, as the inlet temperature increases, both the temperature and pressure of the combustion chamber also increase, and then more PAH will be oxidized at higher temperatures. Second, with the decrease of the equivalence ratio at a fixed inlet air temperature, the PAH quantities increase. This is due to the decrease of the equivalence ratio causing a decrease of the mixture concentration in the cylinder, which makes the engine temperature reduce gradually, and fewer PAHs could be oxidized under lower temperatures.
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ASSOCIATED CONTENT
S Supporting Information *
Aromatic species (Appendix A), reactions for the reduced mechanism (Appendix B), and detailed and reduced mecha1619
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pressure conditions. Proceedings of the U.S. National Combustion Meeting; Ann Arbor, MI, May 17−20, 2009. (14) Kook, S.; Pickett, L. M. Soot volume fraction and morphology of conventional and surrogate jet fuel sprays at 1000-K and 6.7-MPa ambient conditions. Proc. Combust. Inst. 2010, 33 (2), 2911−2918. (15) Golovitchev, V. I. Diesel Surrogate Fuel Mechanism; http://www. tfd.chalmers.se/∼valeri/MECH.html. (16) Tao, F.; Golovitchev, V. I.; Chomiak, J. Application of complex chemistry to investigate the combustion zone structure of DI diesel sprays under engine-like conditions. Proceedings of the 5th International Symposium on Diagnostics and Modeling of Combustion in Internal Combustion Engines (COMODIA 2001); Nagoya, Japan, July 1−4, 2001. (17) Peters, N.; Paczko, G.; Seiser, R.; Seshadri, K. Temperature cross-over and non-thermal runaway at two-stage ignition of nheptane. Combust. Flame 2002, 128 (1−2), 38−59. (18) Andrae, J. C. G.; Brinck, T.; Kalghatgi, G. T. HCCI experiments with toluene reference fuels modeled by a semidetailed chemical kinetic model. Combust. Flame 2008, 155 (4), 696−712. (19) Andrae, J. C. G.; Björnbom, P.; Cracknell, R. F.; Kalghatgi, G. T. Autoignition of toluene reference fuels at high pressures modeled with detailed chemical kinetics. Combust. Flame 2007, 149 (1−2), 2−24. (20) Slavinskaya, N. A.; Frank, P. A modelling study of aromatic soot precursors formation in laminar methane and ethene flames. Combust. Flame 2009, 156 (9), 1705−1722. (21) Tamás, T. Applications of sensitivity analysis to combustion chemistry. Reliab. Eng. Syst. Saf. 1997, 57 (1), 41−48. (22) Gustavsson, J.; Golovitchev, V. I. Spray combustion simulation based on detailed chemistry approach for diesel fuel surrogate model. SAE [Tech. Pap.] 2003, DOI: 10.4271/2003-01-1848. (23) Fredriksson, J.; Bergman, M.; Golovitchev, V. I.; Denbratt, I. G. Modeling the effect of injection schedule change on free piston engine operation. SAE [Tech. Pap.] 2006, DOI: 10.4271/2006-01-0449. (24) Golovitchev, V. I.; Calik, A. T.; Montorsi, L. Analysis of combustion regimes in compression ignited engines using parametric Φ−T dynamic maps. SAE [Tech. Pap.] 2007, DOI: 10.4271/2007-011838. (25) Machrafi, H.; Cavadias, S.; Gilbert, P. An experimental and numerical analysis of the HCCI auto-ignition process of primary reference fuels, toluene reference fuels and diesel fuel in an engine, varying the engine parameters. Fuel Process. Technol. 2008, 89 (11), 1007−1016. (26) Kee, R. J.; Rupley, F. M.; Miller, J. A. CHEMKIN Release 4.1, Reaction Design; San Diego, CA, 2006. (27) Ciezki, H. K.; Adomeit, G. Shock-tube investigation of selfignition of n-heptane−air mixtures under engine relevant conditions. Combust. Flame 1993, 93 (4), 421−433. (28) Davidson, D. F.; Gauthier, B. M.; Hanson, R. K. Shock tube ignition measurements of iso-octane/air and toluene/air at high pressures. Proc. Combust. Inst. 2005, 30 (1), 1175−1182.
nisms in CHEMKIN format with associated thermodynamic data. This material is available free of charge via the Internet at http://pubs.acs.org.
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AUTHOR INFORMATION
Corresponding Author
*Telephone: 86-23-65102473. E-mail: zhengzhaolei2002@ yahoo.com.cn. Notes
The authors declare no competing financial interest.
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ACKNOWLEDGMENTS The research is supported by the National Natural Science Foundation of China (NSFC) through its Project 51006128.
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NOMENCLATURE PAH = polycyclic aromatic hydrocarbon HCCI = homogeneous charge compression ignition CFD = computational fluid dynamics PM = particulate matter NOx = nitrogen oxides CN = cetane number
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REFERENCES
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