New Reduced Chemical Mechanism for Homogeneous Charge

Energy & Fuels 2008, 22, 935–944

935

New Reduced Chemical Mechanism for Homogeneous Charge Combustion Ignition Combustion Investigation of Primary Reference Fuels Chen Huang,* Xingcai Lu, and Zhen Huang Key Laboratory for Power Machinery and Engineering of M.O.E., Shanghai Jiaotong UniVersity, Shanghai, 200240, China ReceiVed August 28, 2007. ReVised Manuscript ReceiVed NoVember 4, 2007

A reduced chemical mechanism for primary reference fuels (PRFs) and their mixtures has been developed to investigate the homogeneous charge combustion ignition (HCCI) combustion and emissions (uHC, CO, and NOx) processes. The reduced mechanism involves 42 species undergoing 62 reactions and has been validated against experiments under engine relevant conditions. First, the ignition delay time predicted by the reduced mechanism has been compared with shock tube experiments, and good agreement has been obtained for the PRF100, PRF90, PRF80, PRF60, and PRF0 fuels over the temperature range 667–1250 K at stoichiometric conditions. Second, the ignition delay time and burn rate has been validated against a rapid compression machine over the equivalence ratio range 0.2–0.5 and the initial temperature range 305–341 K for PRF90 fuel. Third, the ignition delay times and NOx emissions have been compared with HCCI engine experiments and good agreement has been obtained for the PRF90, PRF75, PRF50, and PRF25 fuels.

1. Introduction Homogeneous charge combustion ignition (HCCI) is a promising combustion mode that can achieve high efficiency and low emissions simultaneously, and it is also a key strategy to meet future emission regulations. Since HCCI combustion is controlled primarily by chemical kinetics,1 the focus has been on the development of chemical mechanisms, which can accurately describe the hydrocarbon oxidation process. Various kinds of chemical mechanisms have been developed including detailed and reduced mechanisms. More recently, chemical mechanisms have been coupled with multizone models or multidimensional computational fluid dynamics (CFD) models to consider the mixture inhomogeneity, heat transfer, and turbulence chemistry interaction (TCI)2,3 in the HCCI combustion process. However, a coupled CFD and detailed chemistry model requires substantial memory and CPU time, which is beyond the capability of current computers.4 Thus, a reduced mechanism is required to simulate the HCCI combustion and emissions processes in the multidimensional CFD model. The vast majority of transportation fuels comprise linear and branched hydrocarbons, and much emphasis has been placed on the study of n-heptane and iso-octane. The reason is that these alkane molecules are primary reference fuels (PRFs) for octane rating in spark ignition engines, and their octane numbers * Corresponding author. Tel.: +86-21-34206859. Fax: +86-21-34205553. E-mail address: [email protected]. (1) Pitz, W. J.; Cernansky, N. P.; Dryer, F. L.; Egolfopoulos, F. N. SAE Tech. Pap. Ser. 2007, 2007-01-0175. (2) Aceves, S. M.; Flowers, D. L. SAE Tech. Pap. Ser. 2004, 2004-011910. (3) Kong, S. C.; Reitz, R. D. Combust. Theory Model. 2003, 7, 417– 433. (4) Maroteaux, F.; Noel, L. Combust. Flame 2006, 146 (1–2), 246– 267.

are 0 and 100, respectively.5 Chemical kinetics of PRFs and their mixtures is useful in understanding engine knock and the HCCI combustion process. A large number of experimental studies of the oxidation of n-heptane, iso-octane, and PRF mixtures have appeared in the literature.Thesestudiescompriseshocktubeignitionmeasurements,6–9 jet-stirred-flow reactors,10–12 rapid compression machines,13–17 and HCCI engines.18–20 Previous studies focused primarily on near-stoichiometric conditions and moderate pressures, which provided valuable data for validation of chemical mechanisms relevant to gasoline SI engines.5 However, recent studies7,15,18,19 focused on lean mixtures and relative high pressures conditions, (5) Chaos M.; Kazakov A.; Zhao Z. W.; Dryer F. L. A High-Temperature Chemical Kinetic Model for Primary Reference Fuels. www.interscience. wiley.com (accessed 2007). (6) Ciezki, H. K.; Adomeit, G. Combust. Flame 1993, 93 (4), 421–433. (7) Herzler, J.; Jerig, L.; Roth, P. Proc. Combust. Inst. 2005, 30 (1), 1147–1153. (8) Gauthier, B. M.; Davidson, D. F.; Hanson, R. K. Combust. Flame 2004, 139 (4), 300–311. (9) Fieweger, K.; Blumenthal, R.; Adomeit, G. Combust. Flame 1997, 109, 599–619. (10) Ciajolo, A.; D’Anna, A. Combust. Flame 1998, 112 (4), 617–622. (11) Dagaut, P.; Reuillon, M.; Cathonnet, M. Combust. Sci. Technol. 1994, 95, 233–260. (12) Dagaut, P.; Reuillon, M.; Cathonnet, M. Combust. Flame 1995, 101 (1–2), 132–140. (13) Minetti, R.; Carlier, M.; Ribaucour, M.; Therssen, E.; Sochet, L. R. Combust. Flame 1995, 102 (3), 298–309. (14) Griffiths, J. E.; Halford-Maw, P. A.; Rose, D. J. Combust. Flame 1993, 95 (3), 291–306. (15) Tanaka, S.; Ayala, F.; Keck, J. C.; Heywood, J. B. Combust. Flame 2003, 132, 219–23. (16) Griffiths, J. F.; Halford-Maw, P. A.; Mohamed, C. Combust. Flame 1997, 111, 327–337. (17) Callahan, C. V.; Held, T. J.; Dryer, F. L.; Minetti, R.; Ribaucour, M.; Sochet, L.; Faravelli, R. T.; Gaffuri, P.; Ranzi, E. Proc. Combust. Inst. 1996, 26, 739–746. (18) Lu, X. C.; Chen, W.; Huang, Z. Fuel 2005, 84 (9), 1074–1083. (19) Lu, X. C.; Chen, W.; Huang, Z. Fuel 2005, 84 (9), 1084–1092. (20) Lu, X. C.; Ji, L. B.; Zu, L. L.; Hou, Y. C.; Huang, C.; Huang, Z. Combust. Flame 2007, 149, 261–270.

10.1021/ef700515f CCC: $40.75  2008 American Chemical Society Published on Web 01/19/2008

936 Energy & Fuels, Vol. 22, No. 2, 2008

Huang et al.

Table 1. Initial and Adjusted Values of the Kinetic Parameters for the Reduced Mechanism kinetic parameters

initial value

adjusted value

A5 A12

2.34E+11 1.00E+16

9.00E+10 5.00E+15

which are more suitable for validation of chemical mechanisms relevant to HCCI combustion conditions. With those experimental data available, various kinds of chemical mechanisms describing the chemical behavior of primary reference fuels have been developed over recent years. As far as the complexity of the reaction schemes are concerned, there are generally four categories of chemical mechanisms: detailed, reduced, skeletal, and global. The most comprehensive mechanism is the detailed mechanism, and it is used to understand fully the fundamental chemical processes involved in fuel oxidation. The detailed mechanism is applicable over a wide range of conditions, and it is often used as guidance in development of reduced mechanism. Curran et al.21–23 generated a series of detailed chemical mechanisms to describe the autoignition phenomena and intermediate species of n-heptane, iso-octane, and PRF mixtures at elevated pressures. Battin-Leclerc et al.24,25 developed a computer package (EXGAS) to automatically generate detailed kinetic models for the gas-phase oxidation and combustion of normal and branched alkanes, including n-butane, n-octane, n-decane, n-hexadecane, n-heptane, iso-octane, and PRF mixtures. Ranzi et al.26,27 presented two semidetailed kinetic models for the oxidation of n-heptane and iso-octane, respectively. The second type of mechanism which contains only the most critical elements and reactions of the detailed mechanism is the reduced mechanism.28 More recently, the focus has been on the development of reduced mechanism, because it is feasible for current computers to process the coupling of a multidimensional CFD model and reduced mechanism. A reduced mechanism was developed by Chaos et al.5 to describe the high temperature oxidation and pyrolysis of n-heptane, iso-octane, and their mixtures. A 26-step mechanism for n-heptane was developed by Maroteaux et al.4 based on the Golovithcev n-heptane mechanism,29 and a reduced mechanism containing 29 reactions and 27 species was constructed by Machrafib et al.30 from the Golovithcev iso-octane mechanism29. Similar to the reduced mechanism in size is the skeletal mechanism. Not the details but the skeleton of the reactions is focused in the skeletal mechanism.31 As a result, the reactions (21) Curran, H. J.; Gaffuri, P.; Pitz, W. J.; Westbrook, C. K. Combust. Flame 1998, 114, 149–177. (22) Curran, H. J.; Pitz, W. J.; Westbrook, C. K.; Callahan, C. V.; Dryer, F. L. Symp. (Int.) Combust. 1998, 1, 379–387. (23) Curran, H. J.; Gaffuri, P.; Pitz, W. J.; Westbrook, C. K. Combust. Flame 2002, 129 (3), 253–280. (24) Buda, F.; Bounaceur, R.; Warth, V. Combust. Flame 2005, 142, 170–186. (25) Glaude, P. A.; Conraud, V.; Fournet, R.; Battin-Leclerc, F.; Come, G. M.; Scacchi, G.; Dagaut, P.; Cathonnet, M. Energy Fuels 2002, 16 (5), 1186–1195. (26) Ranzi, E.; Faravelli, T.; Gaffuri, P. Combust. Flame 1997, 108, 24–42. (27) Ranzi, E.; Gaffuri, P.; Faravelli, T. Combust. Flame 1995, 103, 91–106. (28) Zheng J. C. A Study of Homogeneous Ignition and Combustion Processes in CI, SI, and HCCI Engine Systems. Ph.D. Thesis. Drexel University, Philadelphia, PA, 2005. (29) Golovitchev, V L. http://www.tfd.chalmers.se/∼ valeri/MECH.html, Chalmers Univ of Tech, Gothenburg, Sweden, 2000. (30) Machrafib, H.; Lombaertb, K.; Cavadiasa, S.; Guibertb, P.; Amourouxa, J. Fuel 2005, 84 (18), 2330–2340. (31) Jia, M.; Xie, M. Z. Fuel 2006, 85 (17–18), 2593–2604.

are greatly generalized. It is also a competitive type of mechanism for implementation into a multidimensional CFD model. The Shell model is one of the earliest skeletal models, and it is successfully applied in predicting knock in SI engine and autoignition in diesel engines.32 However, it can not properly represent the low temperature oxidation chemistry. In order to resolve this issue, Cox et al.33 extended the Shell model into a mechanism consisting of 15 reactions and 10 species. Hu et al.34 further developed a skeletal mechanism based on the Cox model, and it consisted of 18 reactions and 13 species. By contrast to the Cox model, the exothermicity was computed from the enthalpy change in each elementary reaction. However, cumulative heat release, preignition, fuel consumption, and key species concentrations were not modeled very well. To resolve this issue, an extended reduced chemical mechanism was developed by Li et al.35 This model consisted of 29 reactions with 20 active species, and it reproduced the ignition delay and the preignition heat release within 15%. Although the Li model can accurately reproduce the low temperature behavior of alkanes, it can not describe high temperature reactions properly according to the experiments from Zheng et al.36 Fortunately, a unified model for alkanes was developed by Griffiths et al.37 on the basis of their long-term investigation. According to a sensitivity analysis, the core reactions in the model are the reasonable minimum representation for the C1, C2, and C3 containing species and other important reactions of O, H, OH, and HO2.38 Zheng et al.39 further incorporated the Li model with the Griffiths model to describe the low, intermediate, and high temperature chemistry. This model consisted of 69 reactions and 45 species and was generally in good agreement with experimental data for preignition behavior, ignition delay time, and combustion rate for PRF20 fuel in HCCI engines. Similar to the work of Zheng, Tanaka et al.40 incorporated the Hu model with additional elementary and global reactions for primary reference fuels. Global breakdown reactions for the conversion of intermediates alkanedione and olefin into CO and HO2 as well as an interaction reaction between n-heptyl and iso-octane were included. The model reproduced the ignition delay time and burn rate obtained in the rapid compression machine experiments for PRF90 over a wide range of conditions. Jia et al.31 offered a skeletal mechanism for iso-octane HCCI combustion process based on the Tanaka mechanism. The skeletal mechanism was validated against experiments including a shock tube, rapid compression machine, jet-stirred reactor, and HCCI engine. Huang et al.41 developed a reduced mechanism for n-heptane oxidation process based on the Li and Griffiths model. In addition, the kinetic parameters of the key reactions in the mechanism were adjusted by using a genetic algorithm optimization methodology to improve predicted ignition delay times. The last category of mechanism is the global mechanism, and it describes the chemistry in terms of several principal (32) Halstead, M. P. Combust. Flame 1977, 30, 45–60. (33) Cox, R. A.; Cole, J. A. Combust. Flame 1985, 60 (2), 109–123. (34) Hu, H.; Keck, J. C. SAE Tech. Pap. Ser. 1987, 872110. (35) Li, H.; Miller, D. L.; Cernansky, N. P. SAE Tech. Pap. Ser. 1996, 960498. (36) Zheng J. C.; Yang W. Y.; Miller D. L.; et al. SAE Tech. Pap. Ser. 2001, 2001-01-1025. (37) Griffiths, J. F.; Hughes, K. J.; Schreiber, M.; et al. Combust. Flame 1994, 99, 533–540. (38) Poppe, C.; Schreiber, M.; Griffiths, J. F. Modeling of n-heptane auto-ignition and validation of the results. Joint Meeting of the British and German Sections of the Combustion Institute, Cambridge, 1992; p 360. (39) Zheng, J. C.; Yang, W. Y; Miller, D. L.; et al. SAE Tech. Pap. Ser. 2002, 2002-01-0423. (40) Tanaka, S.; Ayala, F.; Keck, J. C. Combust. Flame 2003, 133, 467– 481. (41) Huang, H. Z.; Su, W. H. Fuel 2005, 84 (9), 1029–1040.

HCCI InVestigation of Reference Fuels

Energy & Fuels, Vol. 22, No. 2, 2008 937 Table 2. Reduced Model for PRFsa Species

C7H16 C7KET21 C8H17OO C8H16 C3H5 CO HCO HO2 N

C7H15–2 C5H11CO C8H16OOH C6H13 C3H4 CO2 H2O H2O2 N2O

C7H15O2 C5H11 OOC8H16OOH C6H13CO C2H5 CH3 O OH NO

C7H14O2HO2 C8H17 OC8H15O C3H6 C2H3 CH2O H2 N2

C7H14O2H C8H18 OC8H15OOH C4H8 C2H4 CH3O O2 H NO2

Reactions (k ) ATb exp(-E/RT)) A 1 2 3 4 5 6 7 8 9 10 11

C7H16 + O2 ) C7H15–2 + HO2 C7H16 + OH ) C7H15–2 + H2O C7H15–2 + O2 ) C7H15O2 C7H15O2 ) C7H14O2H C7H14O2H + O2 ) C7H14O2HO2 C7H14O2HO2 ) C7KET21 + OH C7KET21 ) C5H11CO + CH2O + OH C5H11CO ) C5H11 + CO C5H11 ) C2H5 + C3H6 C7H15–2 ) CH3 + 2C3H6 C7H15–2 ) C2H5 + C2H4 + C3H6

12

C8H18 + O2 ) C8H17 + HO2 reverse Arrhenius coefficients C8H17 + O2 ) C8H17OO reverse Arrhenius coefficients C8H17OO ) C8H16OOH reverse Arrhenius coefficients C8H16OOH + O2 ) OOC8H16OOH reverse Arrhenius coefficients OOC8H16OOH ) OC8H15OOH + OH C8H18 + OH ) C8H17 + H2O C8H17 + O2 ) C8H16 + HO2 reverse Arrhenius coefficients OC8H15OOH ) OC8H15O + OH OC8H15O + O2 ) C2H3 + 2CH2O + C3H4 + CH3 + HO2 C8H17 ) C4H8 + C3H6 + CH3 C8H16 ) C4H8 + C3H5 + CH3 C4H8 + O2 ) C2H3 + C2H4 + HO2

13 14 15 16 17 18 19 20 21 22 23

27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

HO2 + HO2 ) H2O2 + O2 H2O2 + M ) OH + OH + M O2 + H ) OH + O reverse Arrhenius coefficients OH + OH ) HO2 + H H + O2 + M ) HO2 + M HO2 + OH ) H2O + O2 OH + H + M ) H2O + M O + O + M ) O2 + M CO + OH ) CO2 + H CO + HO2 ) CO2 + OH CO + O + M ) CO2 + M O2 + CO ) O + CO2 HCO + O2 ) CO + HO2 HCO + M ) CO + H + M CH2O + OH ) HCO + H2O CH2O + HO2 ) HCO + H2O2 CH3 + HO2 ) CH3O + OH CH3O + O2 ) CH2O + HO2 C2H3 + O2 ) CH2O + HCO C2H4 + OH ) C2H3 + H2O C2H4 + O ) CH3 + HCO C2H5 + O2 ) C2H4 + HO2 C3H4 + OH ) CH2O + C2H3 C3H5 + O2 ) C3H4 + HO2 C3H6 + OH ) C3H5 + H2O C3H6 + O2 ) C3H5 + HO2 C3H6 + HO2 ) C3H5 + H2O2

51 52 53

N + NO ) N2 + O N + O2 ) NO + O N + OH ) NO + H

24 25 26

b

E

n-heptane 2.80E+14 4.80E+09 2.00E+12 6.00E+11 9.00E+10 2.97E+13 1.00E+16 1.00E+11 3.20E+13 3.00E+13 1.20E+13

0 1.3 0 0 0 0 0 0 0 0 0

47180 690.5 0 20380 0 26700 42400 9600 28300 29800 29600

5.00E+15 1.00E+12 1.00E+12 2.51E+13 1.14E+11 1.00E+11 3.16E+11 2.51E+13 8.91E+10 1.00E+13 3.16E+11 3.16E+11 3.98E+15 2.45E+13

0 0 0 0 0 0 0 0 0 0 0 0 0 0

46000 0 0 27400 22400 11000 0 27400 17000 3000 6000 19500 43000 32000

1.28E+12 1.92E+12 2.00E+14

0 0 0

49000 48000 35900

2.00E+12 7.59E+16 2.63E+16 1.45E+13 3.72E+04 2.82E+18 3.01E+15 2.19E+22 1.20E+17 4.79E+07 1.50E+14 1.82E+10 2.50E+12 1.35E+13 1.86E+17 2.43E+10 3.00E+12 4.30E+13 7.59E+10 3.98E+12 3.63E+06 1.26E+07 8.32E+11 1.00E+12 6.03E+11 3.09E+06 1.95E+12 3.16E+11

0 0 -0.7 0 2.4 -0.9 0 -2 -1 1.2 0 0 0 0 -1 1.2 0 0 0 0 2 1.8 0 0 0 2 0 0

0 46000 17040 700 -2110 0 17330 0 0 70 23600 2385 47800 400 17000 -447 8000 0 2700 -250 2500 220 3875 0 10000 -300 39040 14900

3.50E+13 2.65E+12 7.33E+13

0 0 0

iso-octane

high temperature

NOx 330 6400 1120


Huang et al. Table 2. Continued

54 55 56 57 58 59 60 61 62 a

N2O + O ) N2 + O2 N2O + O ) NO + NO N2O + H ) N2 + OH N2O + OH ) N2 + HO2 N2O + M ) N2 + O + M NO + HO2 ) NO2 + OH NO2 + O ) NO + O2 NO2 + H ) NO + OH NO + O + M ) NO2 + M

1.40E+12 2.90E+13 4.40E+14 2.00E+12 1.30E+11 2.11E+12 3.90E+12 1.32E+14 1.06E+20

0 0 0 0 0 0 0 0 -1.4

10810 23150 18880 21060 59620 -480 -240 360 0

Units: A mol cm s K, E cal/mol.

Figure 1. Schematic diagram for the branching pathways of low and intermediate temperature regions for hydrocarbon oxidation.22

mechanism involves 42 species undergoing 62 reactions and has been validated against experiments under engine relevant conditions. First, the ignition delay time predicted by the reduced mechanism has been compared with shock tube experiments and good agreement has been obtained for the PRF100, PRF90, PRF80, PRF60, and PRF0 fuels over the temperature range 667–1250 K at stoichiometric conditions. Second, the ignition delay time and burn rate has been validated against a rapid compression machine over the equivalence ratio range 0.2–0.5 and the initial temperature range 305–341 K for PRF90 fuel. Third, the ignition delay times and NOx emissions have been compared with the recent HCCI engine experiments in our laboratory, and good agreement has been obtained for the PRF90, PRF75, PRF50, and PRF25 fuels. 2. Mechanism Development See Table 2 for full details of the mechanism.

Figure 2. Comparison of experimental ignition delay times9 with computed values using reduced mechanism for Φ ) 1, p ) 40 bar.

species in one or more overall functional relations.28 Global mechanisms have successfully described high temperature chemistry. However, it is hard to include low and intermediate chemistry in such a simplified model. Zheng et al.42 presented a seven-step global mechanism to simulate the HCCI combustion process. The model included five reactions to represent degenerate chain branching in the low temperature region and two reactions for the high temperature region. A four-reaction model and a five-reaction model were developed by Müller et al.43 and Schreiber et al.,44 respectively. These two global models were only used to predict ignition delays, and they are not suitable for prediction of the full HCCI behavior that occurs with PRF fuels.28 In this paper, a reduced chemical mechanism has been developed to study the HCCI combustion process of primary reference fuels with different octane numbers. The reduced (42) Zheng, J. C.; Miller, D. L.; Cernansky, N. P. SAE Tech. Pap. Ser. 2004, 2004-01-2950. (43) Müller, C. U.; Peters, N.; Linˇán, A. Proc. Combust. Inst. 1992, 24, 777–784. (44) Schreiber, M.; Sakak, S. A.; Lingens, A.; Griffiths, F. J. Proc. Combust. Inst. 1994, 25, 933–940.

2.1. Low and Intermediate Temperature Reactions. The low and intermediate temperature oxidation of n-heptane and iso-octane is quite similar. A schematic diagram for the branching pathways of low and intermediate temperature regions for hydrocarbon oxidation is shown in Figure 1.22 The majority of the alkanes mechanism pathways are in agreement with this diagram. However, the experimental work by Ciajolo et al.10 shows that there are still some discrepancies on the amount of intermediate species for n-heptane and iso-octane during the low temperature reaction regions. For the low temperature oxidation of n-heptane, more aldehydes in the products are found. This is due to easy-to-abstract H-atoms in linear alkanes like n-heptane. Thus, the degenerate chain-branching path is favored. For the low temperature oxidation of iso-octane, cyclic ethers and conjugate olefins are dominant products. This is due to a lack of H-atoms for internal abstraction of iso-octane. Thus, it limits the degenerate chain-branching route and favors the propagation path toward species like cyclic ethers and conjugate olefins. On one hand, the low and intermediate temperature oxidation reactions for n-heptane are derived from the work of Marotreaux et al.,4 and they consist of 11 reactions. This submechanism has been compared with the Curran and Golovithchev mechanisms21,29 and shock tube experiments of Ciezki et al.6 at constant and variable volume simulations. In addition, the kinetic parameter of the second O2 addition reaction R5 in the reduced model is adjusted to improve predicted ignition delay times in shock tube experiments, and the adjusted value of the kinetic parameter is shown in Table 1. On the other hand, the low and intermediate temperature oxidation reactions for iso-octane are derived from the work of Jia et al.,31 and they include 12 reactions. This submechanism is extended from the Tanaka40 model, and it has been validated against experiments including a shock tube, rapid compression machine, jet-stirred reactor, and HCCI engine. The high carbon hydrocarbon breaking into low carbon hydrocarbon is ignored in the Tanaka model, so the following reactions are included in the Jia model to link the products during the low temperature region with the high temperature region.


Energy & Fuels, Vol. 22, No. 2, 2008 939

OC8H15O + O2 ) C2H3 + 2CH2O + C3H4 + CH3 + HO2 (R20) C8H17 ) C4H8 + C3H6 + CH3

(R21)

C8H16 ) C4H8 + C3H5 + CH3

(R22)

C4H8 + O2 ) C2H3 + C2H4 + HO2

(R23)

Moreover, the kinetic parameter of the initiation reaction R12 in the reduced model is adjusted to improve predicted ignition delay times in shock tube experiments, and the adjusted value of the kinetic parameter for the reduced model is shown in Table 1. 2.2. High Temperature Reactions. The submodel for high temperature reactions is based on the core reactions of Griffiths model,14 including 27 reactions. Several modifications to the core reactions of Griffiths model are made as follows. According to Zheng and Huang,39,41 two irreversible reactions in the model of Griffiths are merged together as one reversible reaction. H + O2 S O + OH

(R26)

Zheng39 substituted two elementary reactions in the Griffiths model, C3H4 + OH ) C2H3 + CH2O

(R46)

C3H5 + O2 ) C3H4 + HO2

(R47)

(R51)

N2O + O ) NO + NO

(R55)

N2O + M ) N2 + O + M

(R58)

Bowman49

investigated the NOx emission behavior for a diesel engine at low temperautre and lean mixture conditions. Experimental results indicate that the majority of NOx is N2O. Moreover, calculations in HCCI engines made by Amne´us et al.50 indicated that for moderate NOx levels, N2O reactions played an important role in the NOx formation. Thus, the N2O reactions in Golovitchev iso-octane mechanism are included. 2.3.2. NO2 Mechanism. High proportions of NO2 were found by Amne´us et al.50 in some HCCI engines. This is due to high temperature inhomogeneities, poor mixing, and slow overall combustion. NO is most likely to be converted into NO2 by peroxy radicals formed from hydrocarbon combustion reactions. NO + HO2 ) NO2 + OH

(R59)

Thus, the NO2 reactions in the Golovitchev iso-octane mechanism are included. Besides, for the HCCI conditions investigated, prompt NOx formation is not relevant except for fuel-rich combustion.

3. Comparison of the Reduced Mechanism with Experimental Data

for the global reaction, C3H5 + O2 ) CH2 + 2HCO + OH Both experiments and modeling show conclusively that the principal high pressure, high temperature destruction route of the methyl radical (CH3) is via CH3 + HO2 ) CH3O + OH. This reaction is much faster than the CH3 + O2 reaction and leads to significant exothermicity and branching leading to hot ignition.45 So CH3 + O2 ) CH2O + OH in the Griffiths model is substituted by CH3 + HO2 ) CH3O + OH (R40). Besides, CH3O + O2 ) CH2O + HO2 (R41) is included for a new intermediate species CH3O decomposing. Reaction (R35) from the standard set for the CO oxidation46 is added to this submodel to properly describe CO oxidation chemistry, CO + O2 ) CO2 + O

N + NO ) N2 + O

(R35)

Elementary reactions relevant to species CH3CHO or CH3CO are neglected, since these species are not included in the products of low and intermediate temperature reactions of the reduced mechanism. Four elementary reactions relevant to CH2O oxidation are substituted by the following reactions in the Marotreaux mechanism, CH2O + OH ) HCO + H2O

(R38)

CH2O + HO2 ) HCO + H2O2

(R39)

2.3. NOx Mechanism. The submechanism for NOx formation is derived from Golovithcev iso-octane mechanism,29 including 12 reactions. In gasoline or diesel engines where the flame temperature is high, typically around 2700 K, results show that the Zeldovich NO mechanism is sufficient to describe the total NOx. However, when it comes to the HCCI combustion mode where the temperture is relatively lower with rather high pressure, reaction pathways other than thermal NO are needed. 2.3.1. N2O Mechanism. Wolfrum et al.47 and Malte et al.48 proposed the following N2O/NO reactions at temperature below 1800 K, whereas the thermal NO is more dominate above that temperature. (45) Choi, M. Y.; Dryer, F. L.; Haggard, J. B., Jr.; Borowski, B. Observations of a Slow Burning Regime for Hydrocarbon Droplets: n-Heptane/Air Results. In Twenty-Third Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1990; pp 1597– 1604. (46) http://www.me.berkeley.edu/gri_mesh/version30/text30.html (accessed 2000). (47) Wolfrum, J. Chem.-Ing.-Tech. 1972, 44, 656. (48) Malte, P. C.; Pratt, D. T. Combust. Sci. Technol. 1974, 9, 221.

In this section, comparison of the reduced mechanism with Curran mechanism and shock tube ignition delay measurements has been done. Furthermore, the reduced mechanism is validated against the Tanaka mechanism and rapid compression machine experiments. Finally, comparison is made between the reduced mechanism and recent HCCI engine experiments in our laboratory. 3.1. Shock Tube Ignition Delay Measurements. Fieweger et al.9 investigated the self-ignition behavior of primary reference fuels under engine relevant conditions by the shock tube technique. The experiments were conducted for a pressure of 40 ( 2 bar and a stoichiometric equivalence ratio. A comparison of the reduced model, Curran mechanism,21 and experimental data is made under constant volume conditions. The ignition delay time is defined as the time from the beginning of calculation until the pressure increases. Figure 2 displays a comparison of computed ignition delay times against shock tube experimental data for PRF100, PRF90, PRF80, PRF60, and PRF0 fuels at stoichiometric equivalence ratios with an initial pressure of 40 bar. Overall, good agreement between computational and experimental results for primary reference fuels with different octane numbers is obtained. However, there are still some discrepancies between the reduced mechanism and experimental data, especially for pure iso-octane and n-heptane, and they are explained in the next figure. In the high temperature region, 1000 < T < 1250 K, the respective ignition delay times show an approximately linear dependence on the 1/T, and there is no significant difference between the ignition delay times of the primary reference fuel mixtures. In the low and intermediate temperature range, 667 < T < 1000 K, an S-shaped dependence of ignition delay times versus 1/T is observed in Figure 2, which is in good coherence with the experimental trend. It indicates that the reduced mechanism can accurately predict the negative temperature coefficient (NTC) behavior of alkane oxidation. The n-heptane fraction has a big influence on the ignition delay times in this temperature range. (49) Bowman C. T. Control of Combustion-Generated Nitrogen Oxide Emissions: Technology Driven by Regulation. In 24th Symposium (International) on Combustion; The Combustion Institute: Pittsburgh, PA, 1992. (50) Amnéus P.; Mauss F. SAE Tech. Pap. Ser. 2005, 2005-01-0126.


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Figure 3. Comparison of experimental ignition delay times9 with computed values using the reduced mechanism and Curran mechanism23 for Φ ) 1, p ) 40 bar.

The autoignition behaviors of the PRF100, PRF90, PRF80, PRF60, and PRF0 fuels are compared with the shock tube experiments and Curran mechanism, and also with the detailed mechanism. Experiments and simulations are conducted at stoichiometric conditions with an initial pressure of 40 bar, and Figure 3 summarizes the results. For pure primary reference fuels, i.e. iso-octane and n-heptane, shown in Figure 3a and e, the reduced mechanism can predict the NTC behavior, and the ignition delay times are in good agreement with experiments and the detailed mechanism. However, there are still discrepancies between the reduced model and the experimental results. In Figure 3a, for pure iso-octane, the difference between the

reduced mechanism and the experimental results is big in the 1000/T range 1.1–1.2. The reason for this is that the reduced mechanism during this temperature range, the olefin formation pathway, is more favored than the first O2 addition reaction pathway. The olefin is a rather stable species in the oxidation process, which finally leads to the delay of ignition timing. While on the contrary, for the reduced mechanism in the 1000/T range 1.2–1.3, the chain branching pathway is as important as the olefin formation pathway, which leads to the advance of ignition timing. In Figure 3e, for pure n-heptane, there is a difference between the reduced mechanism and the experimental results in the 1000/T range 0.8–1.0. The reason is that for the



Figure 4. Comparison of calculated values using the reduced mechanism and Tanaka mechanism against rapid compression machine experiments.40

Figure 5. Definition of ignition delay times.

reduced mechanism in the temperature range, the n-heptyl molecules are mainly consumed through the β-decomposition pathway. The delay of ignition timing is due to the slow reaction rate of the β-decomposition pathway. For PRF mixtures, i.e. PRF90, PRF80, and PRF60, shown in Figure 3b-d, the ignition delay times predicted by the reduced mechanism are in good agreement with experiments and detailed mechanism. In the low temperature region, 667 < T < 800 K, the detailed mechanism overpredicts the ignition delay times when compared with the experimental data, and the ignition delay times predicted by the reduced mechanism are closer to the experimental data. In the high temperature region, 940 < T < 1050 K, the reduced mechanism overpredicts the ignition delay times when compared with the experimental data, and the ignition delay times predicted by the detailed mechanism are closer to the experimental data. 3.2. Rapid Compression Machine. A rapid compression machine has been used by Tanaka et al.40 to study the effects

of fuel structure on the HCCI combustion process as well as offering some fundamental data for the mechanism validation of primary reference fuels. The definition of ignition delay time and burn rate for a rapid compression machine are employed according to the method proposed by Tanaka. A comparison of calculated values using the reduced mechanism and Tanaka mechanism against rapid compression machine experiments is shown in Figure 4. The effects of the equivalence ratio on the ignition delay time and burn rate for PRF90 are investigated, and the initial temperature and pressure are 318 K and 1 bar, respectively. Figure 4 a and b show good agreement between the reduced model and experimental results. However, the influence of the equivalence ratio on ignition delay times predicted by the reduced mechanism is not as obvious as that in the Tanaka mechanism and experimental data. We can also obtain this phenomenon in Figure 4b. For a high equivalence ratio, i.e. Φ ) 0.5, the burn rates predicted by the reduced model and Tanaka model are slower than experimental data. For a low equivalence ratio, i.e. Φ ) 0.2, the burn rates predicted by the reduced model and Tanaka model are quicker than experimental data. Furthermore, the effect of the initial temperature on ignition delay times is shown in Figure 4c. The ignition delay times predicted by the Tanaka mechanism depend strongly on the initial temperature. However, this phenomenon is not that obviously predicted by the reduced mechanism and experimental data. Although there is a slight discrepancy on ignition delay times and burn rates between the reduced mechanism and experimental data, the trend is well reproduced. 3.3. HCCI Engine. Our laboratory has been doing fundamental research for several years on HCCI combustion with primary reference fuels and their mixtures.18–20 A four-cylinder high-speed direct injection (DI) diesel engine was reformed. One cylinder was running with the HCCI mode, and the others


Huang et al.

Figure 6. Comparison of pressure profiles calculated by the reduced model against HCCI engine experiments for PRF25, PRF50, PRF75, and PRF90.



Figure 7. Comparison of ignition delay times calculated by the reduced mechanism against engine experimental data.

Figure 8. Comparison of NOx emission calculated by the reduced mechanism against engine experimental data.

were running with the original DI diesel engine mode. Two sets of injection pumps were installed on the prototype engine to supply two kinds of fuels, and the test fuel was injected into the intake pipe to get the homogeneous mixture prepared. The experiments were performed using PRF25, PRF50, PRF75, and PRF90 fuels, respectively. The engine speed was 1800 rpm, and the indicated mean effective pressure (IMEP) range was 0.141–0.457 MPa. The effects of different fuels on ignition timing, combustion duration, heat release, cycle-to-cycle variation, and emissions were evaluated. 3.3.1. Definition of Ignition Delay Times. A number of symbols used in this paper are defined in Figure 5. The first stage ignition delay time t1 corresponds to the crank angle when there is a slight pressure rise due to the occurrence of the first stage formation of hydroperoxide H2O2. The second stage

ignition delay time t2 corresponds to the crank angle when the H2O2 decomposition occurs, the OH concentration increases rapidly, and the pressure increases simultaneously. 3.3.2. Comparison between the Reduced Mechanism and HCCI Engine Experiments. Comparisons of pressure profiles calculated by the reduced model against HCCI engine experiments for PRF25, PRF50 PRF75, and PRF90 mixtures at various equivalence ratios are shown in Figure 6. Calculation starts at 128° before top dead center (BTDC), i.e. intake valve close (IVC), and the initial temperature and pressure are calculated by the engine cycle simulation code; they are 373 K and 1.33 atm, respectively. The engine speed is 1800 rpm. Not surprisingly, all the peak pressures predicted by the single-zone model are much higher than the experimental data. This is the major disadvantage of a single-zone model, because it is based on the assumption of perfect mixture homogeneity and no heat transfer is considered. All the fuel ignites simultaneously, which leads to a very fast heat release and steep rise of cylinder pressure. As we can see in Figure 6, for the PRF25, PRF50, PRF75, and PRF90 fuels, the second stage ignition delay time predicted by the reduced mechanism is in good agreement with experimental data generally. This proves that the reduced mechanism is capable of predicting the ignition delay time for various mixtures of n-heptane and iso-octane. However, there are still discrepancies between the reduced mechanism and experimental results. For lean mixture conditions, such as PRF25 Φ ) 0.235, PRF50 Φ ) 0.1936, PRF75 Φ ) 0.2371, and PRF90 Φ ) 0.296, the ignition delay times are later than that predicted by the reduced mechanism. The reason is that, based on a singlezone model, it is hard to consider the mixture inhomogeneity, heat transfer, and in-cylinder turbulence flow. Those factors have


Huang et al.

Figure 9. NOx profiles calculated by the reduced mechanism.

more effect on the lean mixture conditions than the rich mixture conditions. Thus, the ignition delay times predicted by the single-zone model are much earlier than experimental results for lean mixture conditions. Additionally, in order to simulate the actual HCCI combustion process including in-cylinder mixture inhomogeneity, heat transfer, and turbulence interaction, it is a must for the coupling of the chemical mechanism with a multidimensional CFD model. Figure 7 shows the comparison of ignition delay times calculated by the reduced mechanism against engine experimental data. There is no obvious low temperature reaction observed for the PRF90 fuel, so only high temperature ignition delay time is discussed here. The first and second stage ignition delay times predicted by the reduced mechanism are a little earlier than experimental data. The reason for this is that the reduced mechanism is based on the single-zone model, which neglects the mixture stratification, wall heat transfer, and incylinder turbulence flow. The ignition delay times are advanced according to the single-zone model. We need to notice that for the PRF25 fuel with Φ g 0.328, the first and second stage ignition delay times predicted by the reduced mechanism are later than the experimental data. The reason for this is that for a low octane number fuel working under heavy load conditions, i.e. IMEP g 0.437 MPa, engine knock occurs easily, and the ignition delay times advance greatly. The experimental data indicate that for Φ ) 0.328, the dp/dΦ has reached 1.94 MPa/ deg. In that case, the engine is already in knock conditions. One of the most important characteristics of HCCI combustion process is its ultralow NOx emissions, usually lower than 10 ppm. However, it is still necessary to understand the NOx emission mechanism, because the rapid increase in NOx emission is a signal of engine knock. A comparison of NOx emission calculated by the reduced mechanism against engine experimental data is shown in Figure 8. With the increase of equivalence ratio, the measured NOx increases, and this trend is accurately predicted by the reduced model. However, there are discrepancies between the simulated and experimental results for PRF25 and PRF50 fuels at rich mixture conditions, i.e. Φ g 0.32. This is due to the disadvantage of a single-zone model. Besides, the PRF25 and PRF50 fuels are low octane number fuels, which means that autoignition occurs easily for them. The discrepancies in temperature predicted by the reduced model and experimental data are much larger for those fuels. In this way, NOx emission which is mainly dominated by the Zeldovich NO mechanism at high temperature is much higher than experimental data.

NOx profiles calculated by the reduced mechanism at both low and high temperatures are shown in Figure 9. For the low temperature condition, the peak temperature is 1765 K (PRF25, Φ ) 0.235). The majority of NOx emission is N2O whereas the amounts of NO and NO2 are small, shown in Figure 9a. For the high temperature condition, the peak temperature is 2023 K (PRF75, Φ ) 0.354). The majority of NOx emission is NO whereas the amounts of N2O and NO2 are small, shown in Figure 9b. Thus, for the low temperature and high pressure conditions in the HCCI combustion process, NOx mechanisms other than thermal NO are needed. The calculated results are in good agreement with experimental data, and the reason why the modeled results are higher than the experimental data has been explained above. 4. Conclusions This paper presents a reduced chemical mechanism which can be used to study the HCCI combustion process for primary reference fuels with different octane numbers. The reduced mechanism involves 42 species undergoing 62 reactions. The ignition delay times predicted by the reduced mechanism have been compared with Curran mechanism and shock tube experiments, and good agreement has been obtained for the PRF100, PRF90, PRF80, PRF60, and PRF0 fuels over the temperature range 667–1250 K at stoichiometric conditions. The ignition delay times and burn rates have been validated against the Tanaka mechanism and rapid compression machine experiments over the equivalence ratio range 0.2–0.5 and the initial temperature range 305–341 K for the PRF90 fuel. The ignition delay times and NOx emissions have been compared with HCCI engine experiments in our laboratory, and good agreement has been obtained for the PRF90, PRF75, PRF50, and PRF25 fuels. However, because of the disadvantage of a single-zone model, there are still discrepancies between the reduced model and experimental results. In order to simulate the actual HCCI combustion process including mixture inhomogeneity, wall heat transfer, and in-cylinder turbulence interaction, a multidimensional CFD code coupled with the reduced mechanism is going to be applied to study the PRFs HCCI combustion and emissions processes. Acknowledgment. This work is supported by the National Basic Research Program of China (Grant No. 2007CB210007) and Shanghai Nature Science Foundation (Grant No. 06ZR14045). EF700515F

New Reduced Chemical Mechanism for Homogeneous Charge

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