Development of a Reduced Primary Reference Fuel Mechanism for

Article pubs.acs.org/EF

Development of a Reduced Primary Reference Fuel Mechanism for Internal Combustion Engine Combustion Simulations Hu Wang,*,†,‡ Mingfa Yao,‡ and Rolf D. Reitz† †

Engine Research Center, University of WisconsinMadison, 1500 Engineering Drive, Madison, Wisconsin 53705, United States State Key Laboratory of Engines, Tianjin University, No. 92 Weijin Road, Nankai District, Tianjin University, Tianjin 300072, P. R. China

‡

ABSTRACT: A reduced PRF mechanism was proposed for combustion simulations of PRF and diesel/gasoline fuels based on the latest LLNL mechanism. The reduced PRF mechanism consists of 73 species and 296 reactions. The major reaction pathways of the detailed mechanism were mostly retained in the reduced mechanism, which ensures its predictive capability, the ability to be extended to other fuels, and the high computational efficiency of the reduced mechanism. The important reaction pathways and reactions in the reduced mechanism are identified and discussed. Furthermore, the reaction rates of two reactions, HO2 + OH = HO2 + O2 and HO2 + HO2 = H2O2 + O2, in the hydrogen submechanism are discussed and updated. The reduced mechanism was validated with measured ignition delays, laminar flame speeds, premixed flame species concentrations, jet stirred reactor and shock tube species profiles, and PRF fuel HCCI and PPCI combustion and diesel/gasoline direct injection spray combustion data. The reduced mechanism predicts well the ignition timings, flame speeds, and important species concentrations under various validation conditions and shows reliable performance under different engine validation conditions. The overall results suggest that the current mechanism can provide reliable predictions for PRF and diesel/gasoline combustion CFD simulations.

■

INTRODUCTION The internal combustion (IC) engine still remains one of the most important power sources in modern society after over a century of development. However, further improvements in thermal efficiency and reduction in pollutant emissions are still needed because of energy shortages and environmental regulations, which have led to the emergence and development of many novel combustion concepts1,2 that could greatly improve the thermal efficiency and reduce the emissions of IC engines. A common characteristic of these novel combustion concepts is highly premixed low temperature combustion, which highlights the importance of the chemical kinetics, because combustion processes are mainly controlled by fuel oxidation chemistry.3 Currently, computational fluid dynamic (CFD) simulations play an important role in the development of IC engine technologies and in the understanding of the physical and chemical processes behind apparent experimental phenomena. Therefore, reliable chemical reaction mechanisms are the key to understand in-cylinder processes and to guide the design of IC engines. Until now, diesel and gasoline fuels still remain the most important and widely used fuels for IC engines. However, because of the complex compositions of diesel and gasoline fuels, simplified surrogate fuel models are still very useful for representing current transportation fuels. For example, the primary reference fuel (PRF) mechanism is one of the most prevalent models to mimic the combustion processes of diesel and gasoline fuels. The most widely used detailed PRF mechanisms were developed by Curran et al. 4,5 However, the detailed mechanisms contain too many species and reactions (on the © XXXX American Chemical Society

order of 100 to 10 000); thus, it is impossible to directly couple these mechanisms with CFD codes for engine combustion simulations. Therefore, various mechanism reduction methods and many skeletal and reduced mechanisms have been proposed and developed for engine simulations. Patel et al.6 developed a reduced n-heptane mechanism for diesel homogeneous charge compression ignition (HCCI) combustion simulations from a skeletal n-heptane mechanism. On the basis of Patel et al.’s work, Ra and Reitz7 developed a very compact PRF mechanism for IC engine combustion simulations. They proposed highly lumped reaction pathways for the oxidation of PRF fuels, where n-C7H16 and i-C8H18 share common C1−C3 and H2−O2 reaction systems. They also proposed a method to optimize the reaction rates of important reactions to match measured ignition delay data. The reduced mechanism can predict well the combustion processes of diesel and gasoline fuels. Because of the compact size of the mechanism, it has been widely applied for the simulation of direct injection and HCCI, PCCI, and RCCI combustion simulations. Yoo et al.8 proposed a reduced n-heptane mechanism with 58 species from the detailed Lawrence Livermore National Laboratory (LLNL) n-heptane mechanism by using the direct relation graph (DRG), 9 DRG-aided sensitivity analysis (DRGASA),10 quasi-steady-state approximation (QSSA),11 and isomer lumping methods.12 Tanaka et al.13 also constructed a reduced PRF mechanism for lean condition HCCI combustion simulations. The predictions of this mechanism agree reasonably well with data taken from a rapid compression Received: October 3, 2013

A

dx.doi.org/10.1021/ef401992e | Energy Fuels XXXX, XXX, XXX−XXX

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engine. However, as pointed out by Ra et al.,7 the highly lumped one-step reaction from alkyl-ketoperoxide to CO reduced the prediction ability of this mechanism. Besides, the proposed mechanism underpredicts the ignition delay in a shock tube, although it can predict reasonably well the ignition delay in a rapid compression engine. Liu et al.14 constructed a skeletal PRF mechanism by using a semidecoupling methodology. In their study, they first assessed the performance of several available reduced/skeletal PRF mechanisms. They found that three tested skeletal models failed to predict laminar flame speeds. Also the models of Patel6 and Ra7 failed to predict the evolution of CO and CO2 in a jetstirred reactor (JSR). A reduced PRF oxidation mechanism was formulated by combining C0−C1,15 C2−C3,6 and the low temperature and decomposition mechanism.13,16 Generally, the mechanism can provide reasonably good predictions under various validation conditions. Recently, Luong et al.17 proposed a reduced PRF mechanism that is suitable for lean HCCI combustion simulations, which contains 116 species, and validated the mechanism by comparing the ignition delay with the detailed mechanism and predicted flame speeds with available data. Although the mechanism shows good agreement with the detailed one, it is still too large for use in engine combustion simulation for design and optimization. All these reduced/skeletal PRF mechanisms exhibit various advantages and disadvantages under different validation conditions. However, there are some other considerations that should be taken into account. First, these reduced mechanisms are usually formulated by combining submechanisms that come from different sources. This might result in unexpected issues to the mechanism, such as the numerical stiffness problem, and the choice of thermodynamics and transport properties also presents a problem. Second, these mechanisms usually contain highly lumped reactions to keep the mechanism as small as possible. Although the ignition delays of the reduced mechanisms agree well with available measured data by tuning the reaction rates, the nature of the detailed mechanisms will be inevitably be changed. For instance, sometimes the low temperature heat release (LTHR) feature of n-heptane is missed. Third, the lumped and over-reduced mechanism greatly affects the prediction of PAH building blocks (mostly C2−C4 intermediate species) and the formation of polyaromatic hydrocarbon (PAH) molecules; thus, more complementary species and reactions are needed if a PAH mechanism is introduced. Finally, use of bioderived fuels, such as alcohols and biodiesel, in IC engines is of interest. Because the base PRF mechanism is highly reduced, more species and reactions need to be added into the base mechanisms to formulate a mechanism that can be used for biofuels combustion simulations. This not only greatly increases the size of the mechanism but also affects the performance of both the base PRF and the newly added mechanisms in the combined mechanism. In another aspect, it is becoming affordable to apply more detailed skeletal mechanisms in engine CFD simulations because of the development of computer technology. For example, graphics processing units (GPU) have already been applied in CFD simulations and a great increase in speed can be obtained compared to central processing unit (CPU)-only simulations.18 Also, newer chemistry solvers can greatly accelerate the chemistry calculations. For example, Perini19 developed a sparse analytical Jacobian solver, called SpeedChem, for chemistry calculations, which, when compared to a

baseline KIVA CFD code20 coupled with Chemkin II,21 reduced CPU times by 3−4 times without noticeable changes in pressure trace, heat release rate, or emissions, even for relatively small mechanisms. Therefore, a skeletal mechanism that retains the major reaction pathways and prediction capability of the detailed mechanism, while at the same time offering high extension ability and computational efficiency, is desired. In the current study, a new skeletal PRF mechanism is proposed and developed based on the LLNL gasoline surrogate mechanism by Mehl et al.22 The skeletal mechanism is reduced and formulated based on the general reaction pathways of PRF fuels. Direct relation graph with error propagation (DRGEP),23,24 rate of production, sensitivity analysis,25 and reaction pathway analysis methods were applied for the mechanism reduction. Key reactions in the hydrogen submechanism were also analyzed and updated. The mechanism was validated with measured ignition timings, flame speeds, species profiles in premixed burners, JSR and shock tubes, and also PRF fuel HCCI, PCCI, and direct injection combustion engine data.

2. MECHANISM FORMULATION In the current investigation, the detailed gasoline surrogate mechanism of Mehl et al.22 was taken as the base mechanism. This detailed LLNL mechanism has 1389 species and 5935 reactions. Standalone reduced n-C7H16 and i-C8H18 mechanisms were generated by reducing the base detailed mechanism. After this, the two mechanisms were combined to formulate the present PRF mechanism. Cross reactions between n-C7H16 and i-C8H18 were also adopted from the base mechanism. Reaction rates of the key reactions in the H2−O2 reaction system were evaluated and updated, and several reactions that affect the reactivity of the PRF mechanism were optimized to better match the experimental data. 2.1. Mechanism Reduction Method. A DRGEP-based approach was used for the mechanism reduction. Details about the mechanism reduction process can be found in ref 26. The current mechanism was constructed hierarchically, similar to those of the detailed mechanisms, from the C7−C8 reactions that describe the major fuel molecule decomposition process and then the following C2−C4 intermediate reactions and finally the core H2−CO and methanol submechanisms. DRGEP was first used to analyze the mechanism and for the initial reduction; then reaction pathway analysis was used to determine the major reaction pathways, and those unimportant species were removed from the base mechanism. Then the DRGEP reduction was conducted again to eliminate abundant species. This process was conducted iteratively from the higher C7−C8 level reactions to the lower C2−C3 level reactions to ensure that the reduced mechanism still followed the major reaction pathway of the parent detailed mechanism. At the same time, ROP and sensitivity analysis were also applied to determine the important reaction pathways and reactions. After the major reduction, small adjustments and manual reductions may still be necessary to optimize the reduced mechanism. Finally, reaction rates at the C7−C8 level were further optimized to better match the available data. 2.2. n-Heptane Mechanism. A reduced n-C7H16 mechanism with 49 species and 221 reactions was obtained by applying these reduction methods. The major reactions involved in the n-heptane oxidation process are listed in Table 1. B


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Table 1. Major Reactions in the Reduced n-Heptane Mechanism R1 R2 R3 R4 R5 R6 R7 R8 R9

n-C7H16 + OH = C7H15−2 + H2O n-C7H16 + HO2 = C7H15−2 + H2O2 C7H15−2 = p-C4H9 + C3H6 C7H15−2 + O2 = C7H15O2−2 C7H15O2−2 = C7H14OOH2−4 C7H14OOH2−4 + O2 = C7H14OOH2−4O2 C7H14OOH2−4O2 = n-C7ket24 + OH n-C7ket24 = n-C3H7CHO + CH3COCH2 + OH C7H14OOH2−4 = OH + CH3CHO + C5H10

H atom abstraction by OH H atom abstraction by HO2 alkyl radical decomposition Ṙ + O2 = Ṙ O2 Ṙ O2 = Q̇ OOH Q̇ OOH + O2 = O2Q̇ OOH O2Q̇ OOH = keto-hydroperoxide + OH keto-hydroperoxide decomposition Q̇ OOH = olefin + carbonyl + OH

Table 2. Major Reactions in the Reduced Iso-octane Mechanism R10 R11 R12 R13 R14 R15 R16 R17 R18 R19 R20 R21

i-C8H18 = t-C4H9 + i-C4H9 i-C8H18 + OH = a-C8H17 + H2O i-C8H18 + HO2 = a-C8H17 + H2O2 a-C8H17 = i-C4H8 + i-C4H9 a-C8H17 + O2 = a-C8H17O2 a-C8H17O2 = a-C8H16OOH−b a-C8H16OOH−b + O2 = a-C8H16OOH−bO2 a-C8H16OOH−bO2 = i-C8ketab + OH i-C8ketab = i-C3H7CHO + t-C3H6CHO + OH a-C8H16OOH−b = i-C8eterab + OH i-C8eterab + OH = i-C4H8 + i-C3H7CO + H2O i-C8eterab + HO2 = i-C4H8 + i-C3H7CO + H2O2

unimolecular fuel decomposition H atom abstraction by OH H atom abstraction by HO2 alkyl radical decomposition Ṙ + O2 = Ṙ O2 Ṙ O2 = Q̇ OOH Q̇ OOH + O2 = O2Q̇ OOH O2Q̇ OOH = carbonylhydroperoxide + OH carbonylhydroperoxide decomposition Q̇ OOH=cyclic ether + OH cyclic ether reacts with OH and HO2

Figure 1. Ignition delay predictions before and after the reduction. Solid line: detailed, dashed line: reduced. Ignition delay: time at which there is 400 K temperature increase to the initial temperature.

mechanism also follows the general reaction pathways depicted by Curran et al.5 R10 shows high ignition delay sensitivity at >1000 K conditions and can also be used to improve the prediction of iso-octane concentrations in shock tube simulations. The a-C8H17 radical decomposition reaction, R13, also competes with the low temperature branching reactions as the temperature increases. Therefore, similar to n-heptane, R13 and R15 can be used to tune the reactivity of iso-octane at low and intermediate temperatures. Due to the complex molecule structure, the products of the LT branching reactions of iso-octane contain many intermediate species, which requires more species and reactions to describe their further reaction pathways. Therefore, the size of the reduced iso-octane mechanism is bigger than that of the n-heptane mechanism. However, many of these intermediate species are also involved in the oxidation processes of alcohols, such as butanol, which will be helpful for the incorporation of these submechanisms into the present PRF mechanism in the future. Figure 1 shows the comparisons of predicted ignition delays before and after the reduction. In the current investigation, the

The H atom abstraction reactions with HO2 and OH radicals occur at both low and high temperatures. At low temperatures, the C7H15−2 radical undergoes the typical low temperature (LT) branching reaction pathway, R4 to R9, as listed in Table 1, which has been used to explain the negative temperature coefficient (NTC) behavior in the oxidation of long chain hydrocarbons. The C7H15−2 radical decomposition reaction, R3, competes with the LT branching initial reaction, R4, and thus greatly affects the reactivity of n-heptane at low temperatures and in the NTC region. R5, the Ṙ O2 isomerization reaction, can be used to adjust the overall reactivity in the low temperature range because this reaction can be regarded as the limiting reaction of the whole LT branching reaction pathway. The proposed n-heptane mechanism generally follows the reaction pathway discussed by Curran et al.,4 rather than using highly lumped reactions. 2.3. Iso-octane Mechanism. Similarly, the major reactions in the reduced iso-octane mechanism are listed in Table 2. The iso-octane mechanism contains 58 species and 268 reactions. The main reaction pathway of the reduced iso-octane C


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Figure 2. Reaction rates comparison between experimental data and model predictions.

Table 3. Updated Reactions with New Rate Constants for Ignition Delay Prediction (s−1-cm3-cal/mol) no.

reaction

R1 R2 R3 R8 R11 R12

n-C7H16 + OH = C7H15−2 + H2O n-C7H16 + HO2 = C7H15−2 + H2O2 C7H15−2 = p-C4H9 + C3H6 n-C7ket24 = n-C3H7CHO + CH3COCH2 + OH i-C8H18 + OH = a-C8H17 + H2O i-C8H18 + HO2 = a-C8H17 + H2O2

A 1.90 4.00 3.15 5.00 7.50 2.00

CHEMKIN package27 was applied for the constant volume ignition delay and flame simulations. As shown in Figure 1, the reduced mechanism predicts very similar results as compared to that of the detailed mechanism under various conditions (of interest for engine study). 2.4. PRF Mechanism Formulation. Because the two reduced mechanisms were reduced from the same parent mechanism, a reduced PRF mechanism was formulated by combining these two reduced mechanisms. The n-C7H16 and iC8H18 mechanisms share common reactions for species with three carbon atoms and below. After the formulation of the base PRF mechanism, some further reductions were conducted to the intermediate species at the C3 and C4 level, especially for the iso-octane mechanism, because the size of the reduced isooctane mechanism was still relatively large. The major reaction pathways of i-C4H8, i-C4H7, t-C3H6CHO, and i-C3H7CHO were further analyzed and simplified. Also, the reactions at the C3 level, basically from C3H6 to C3H3, were further reduced to reduce the total reaction number in the mechanism. Finally a PRF mechanism with 73 species and 296 reactions was formulated. The cross reactions between n-C7H16 and iC8H18 were adopted from the parent mechanism, and the thermodynamic and transport properties in the reduced mechanism were also taken from the parent LLNL-detailed gasoline surrogate mechanism.22 2.5. Reduced Mechanism Adjustment. In ref 26 it is observed that the reaction HO2 + OH = H2O + O2 greatly affects the reactivity of the system in n-butanol HCCI combustion cases, and many researchers have already studied this important chain termination reaction in the H2−O2 reacting system. Therefore, in the current investigation the reaction rates of the following reactions were considered. R22 :

R23 :

× × × × × ×

106 103 1019 1016 106 103

b

Ea

2.00 3.37 −1.79 0.00 1.80 3.59

−596.0 13720 31360 39000 1431 17160

HO2 + OH = H 2O + O2

important chain termination reaction

R22 is a termination reaction in the H2−O2 system, which converts two HO2 radicals to H2O2 and O2, and then H2O2 undergoes the H2O2 = 2OH reaction, which can be regarded as the indicator of the high temperature ignition. Figure 2 shows comparisons of the reaction rate versus temperature between measured data and simulations. The reaction rates of this reaction in the GRI 3.0 mechanism,28 the LLNL mechanism,22 Sarathy’s butanol mechanism,29 Slavinskaya’s PAH mechanism,30 and Ra’s PRF mechanism7 are plotted and compared with experimental data in Figure 2a. The experimental data are taken from Hong et al.’s work.31 It can be seen that the LLNL mechanism and Sarathy’s butanol mechanism agree well with the two sets of experimental data, and the reaction rates in these two mechanisms are very similar in the temperature of interest (750−1500 K). However, the reaction rates of this reaction in Slavinskaya’s PAH mechanism and Ra’s PRF mechanism are quite different from the experimental data. Therefore, the default reaction rate constants of R22 were adopted. R23 is a very important chain termination reaction, which can greatly affect the overall reactivity. Some experimental data show that this reaction is highly temperature dependent, while others are not. The reaction rates of R23 from the LLNL mechanism, 22 the GRI 3.0 mechanism, 28 Konnov’s H 2 mechanism,32 Hong’s constants,31 and Burke’s constants33 are plotted in Figure 2b. Again the experimental data were taken from ref 31. It is seen that huge differences exist among the mechanisms. It is seen that Hong’s and Burke’s constants agree with the experimental data best, while the default reaction rate constants in the LLNL mechanism are much lower than in the other mechanisms in the low temperature region. Note that the reaction rate of Hong’s is quite close to that of Burke’s constants. Therefore, the original reaction rate constants of R23

HO2 + HO2 = H 2O2 + O2

termination reaction D


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Figure 3. Measured and predicted ignition timings of i-C7H16. Initial pressure is 40 bar. Experimental data are from refs 35−38.

the chemical information of the mixture such that it can be used to partially validate chemical kinetics models developed for the mixture. Lipzig et al.39 measured the laminar flame speed of nC7H16/air and i-C8H18/air mixtures at 1 bar and 298 K. In addition, Huang et al.40 also measured the flame speed of PRF50/air mixtures at 338 K. The CHEMKIN package27 was applied for the laminar flame speed simulations, and comparisons were made between measured and calculated results. Figure 5 shows the laminar flame speeds predictions of measured and calculated results. The current reduced mechanism predicts satisfactory results for all the tested fuels at the different conditions. 3.3. Premixed Flame Species Profiles. Species profiles in premixed flames were also used to validate the proposed mechanism. Again the simulations used the CHEMKIN package,27 and measured temperature profiles were used as the input.41 The initial conditions for flame simulations are shown in Table 4. Table 4 also shows the initial conditions and inputs for JSR and shock tube simulations, which will be discussed in the following sections. Figure 6 shows the comparisons of measured and predicted species profiles in n-C7H16 premixed flame. As shown in Figure 6, the mechanism predicts well the species concentrations for the major species, including CO, CO2, and O2. What is more, the simulated concentrations of acetylene and ethylene, which play important roles in PAH and soot formation processes, all agree quite well with measured results. Figure 7 shows comparisons of measured and calculated species concentration profiles in i-C8H18 flame. The calculated major species and important intermediate species are close to the measured values. Similar to the n-C7H16 case, the species related to PAH and soot formation, namely C2H2, C2H4, and C3H4, agree quite well with the measured data. This means that the current mechanism can be used as a base mechanism for the prediction of PAH formation when a PAH mechanism is incorporated into the mechanism. This not only confirms its prediction capability but also ensures the extension capability of the reduced mechanism. 3.4. Jet-Stirred Reactor n-Heptane Species Profiles. Herbinet et al.42 conducted experimental and modeling investigations of the oxidation of n-heptane in a JSR. The experiments were modeled, and the calculated results were compared with the measured data. The simulations were conducted with the homogeneous reactor model at constant pressure and temperature in the CHEMKIN package.27 Inputs for the simulations are listed in Table 4, and Figure 8 shows the comparisons of experimental and modeling results. Again reasonable agreements between the simulations and the measured results are obtained. As mentioned above, during

were replaced by two duplicated reactions, with the constants taken from Burke et al.33 k 23 = 1.93 × 1020T −2.49 exp−294K/T + 1.21 × 109T1.2 exp658K/T (cm 3 mol−1 s−1)

It was still necessary to make some further adjustments to the reduced mechanism to better match shock tube ignition delay data; thus, several reactions listed in Tables 1 and 2 were updated further. The ignition delay sensitivity analysis method from ref 7 was applied to optimize the mechanism ignition delay predictions. Updated reactions with their forward reaction rates are listed in Table 3. The final proposed mechanism can be downloaded from ref 34.

3. MECHANISM VALIDATION 3.1. Ignition Delay. Comparison of experimental and modeling results of ignition delay for n-heptane are presented in Figure 3. The references from which the experimental data were taken are presented in the figure captions. The predicted ignition delays at two different equivalence ratios agree well with measured validation data. The experimental shock tube ignition delays of the various PRF fuels conducted by Fieweger et al.35 were compared with the predictions of the current mechanism. Figure 4 shows comparisons between measured

Figure 4. Measured and predicted ignition timings of PRF fuels. Initial pressure is 40 bar; equivalence ratio is 1.0. Experimental data are from ref 37.

and calculated results for various PRF fuels at 40 bar. It is seen that the PRF mechanism generally predicts well the ignition delay of various PRF fuels, and the differences between the PRF fuels are captured well. 3.2. Laminar Flame Speeds. The laminar flame speed is an important global combustion parameter, which also contains E


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Figure 5. Comparison between measured and simulated laminar flame speeds of various PRF fuels. Experimental data are from refs 41 and 42.

Table 4. Initial Conditions for Premixed Flames, JSR, and Shock Tube Simulations of n-C7H16 and i-C8H18. composition/mole fraction type

fuel

fuel

premixed flames

n-C7H16 i-C8H18

type

fuel

fuel

JSR

n-C7H16

0.005

N2/Ar

mass flow g/(cm2·s)

P (bar)

Φ

ref

0.7301/N2 0.6797/N2

0.00617 0.00526

1.0 1.0

1.9 1.9

41 41

T [K]

time [s]

P (bar)

Φ

ref

500−1100

2.0

1.06

1.0

42

O2

0.0398 0.2301 0.0423 0.278 composition/mole fraction O2

N2/Ar

0.055 0.94/He composition [ppm]

type

fuel

fuel

O2

N2/Ar

T [K]

time [ms]

P (bar)

Φ

ref

shock tube

i-C8H18

9.47 × 10−5 9.84 × 10−5

1.208 × 10−3 2.375 × 10−3

0.9987/Ar 0.9975/Ar

907−1615 866−1710

1.34−2.61 1.26−2.74

46.6−62.2 46.2−63.5

0.98 0.52

43 43

Figure 6. Comparisons of measured and predicted species profiles in n-C7H16 premixed flame. Experimental conditions are listed in Table 4, and experimental data are from ref 41.

Figure 7. Comparisons of measured and predicted species profiles in i-C8H18 flame. Experimental conditions are listed in Table 4, and experimental data are from ref 41.

the mechanism reduction process, special attention was paid to ensure the accurate prediction of C2H2 because of its

importance in PAH and soot predictions. The simulation confirms the prediction ability of the current mechanism in F


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Figure 8. Comparisons of measured and predicted results in i-C7H16 JSR simulation. Experimental conditions are listed in Table 4, and experimental data are from ref 42.

Figure 9. Comparisons between measured and simulated results in iso-octane shock tube experiments. Experimental conditions are listed in Table 4, and experimental data are from ref 43.

is greatly simplified, the reduced mechanism is still able to capture the major reaction pathways and predict well the profiles of important intermediate species. 3.6. PRF HCCI Combustion. Marriott et al.44 and Tamagna et al.45 performed gasoline HCCI experimental and numerical investigations using a Caterpillar 3401 single-cylinder fourstroke heavy duty engine. Two of the experimental operating conditions were chosen to validate the proposed reduced PRF mechanism. Dempsey et al.46 conducted PRF HCCI experiments on a General Motors 1.9 L light-duty diesel engine. Four different operating conditions with different PRF numbers, intake temperatures, and equivalence ratios were also chosen to validate the mechanism. Sahoo et al.47 conducted partially premixed compression ignition (PPCI) experiments on another GM 1.9 L engine, which were also used as validation data for

C2H4 and C2H2 concentration predictions, although there are some discrepancies in the position of peak values. 3.5. Shock Tube Iso-octane Species Profiles. Malewicki et al.43 conducted experimental study on the oxidation of isooctane in a shock tube. Experiments at two conditions were simulated, and measured data were compared with the simulation results. The initial conditions and compositions of the experiments are listed in Table 4. The initial conditions from the experiments, including the initial pressure, temperature, and residence time, were taken from the reference as the inputs for the CHEMKIN simulations. Figure 9 shows comparisons between measured and simulated results in an iso-octane shock tube with two different equivalence ratios. The iso-octane mechanism in the reduced PRF mechanism is seen to predict well the evolution of the C3 and C2 species. This means that although the reaction pathway G


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CAT engine, and YC6J engine at bottom dead center were about 19 000, 21 000, 8000, and 10 000, respectively. Only close cycle simulations were conducted, i.e., from IVC to EVO. 3.6.1. CAT HCCI Combustion. Figure 11 shows comparisons of measured and predicted pressure and HRR profiles using the current PRF mechanism for the CAT engine HCCI combustion. The current mechanism can predict well the ignition timings and combustion phasing. The evolutions of the HRR and pressure traces also agree quite well with measured data. Figure 12 shows comparisons of measured and predicted pressure and HRR profiles using the current mechanism for the ERC-GM engine HCCI combustion. The low temperature heat release (LTHR) decreases as the PRF number and intake temperature increase. The predicted peak pressures of PRF77 and PRF 87 are slightly higher than those in experiments, which can be attributed to three reasons: (1) HCCI combustion in real engines is not really homogeneous; there are temperature and composition in homogeneities in the combustion chamber, while for the simulations, sector meshes with uniform temperature and composition distributions were applied. (2) There are some experimental uncertainties. (3) The AHRR calculation involves heat transfer, which can introduce some extra uncertainties. Overall, the agreement between the simulations and experiments is good for all the tested cases. 3.7. PRF PPCI Combustion. Figure 13 shows comparisons between simulations and experiments for the Sandia GM engine with PRF25 PPCI combustion at injection pressure of 860 bar. In this experiment, simulated EGR, which consists of 10% O2, 81% N2, and 9% CO2, was applied to mimic heavy EGR operating conditions. The initial and boundary conditions were controlled well during the experiments, and the experiments provide well-established data for model and mechanism validation. The mechanism predicts reasonably well the pressure and heat release rate profiles for PPCI combustion simulation. Although there are some discrepancies in the pressure trace in the early combustion stages, the overall results agree reasonably well with experimental data. 3.8. Diesel/Gasoline Mixture DI Combustion. Finally, conventional direct-injection diesel/gasoline combustion data were simulated and compared with measured experimental data. Fuels with 80% (vol) diesel/20% gasoline (DG20) and 70% diesel/30% gasoline (DG30) were tested in the experiments. Figure 14 shows comparisons between the simulations and experiments for the conventional direct-injection diesel/ gasoline mixture fuels combustion cases. As shown in Figure 14, the major combustion characteristics under current operating conditions can be predicted well by the present mechanism, with EGR rates reaching up to 50%. Not only the combustion timing but also the peak values of both pressure and HRR are captured well by the simulations. This further confirms the prediction capability of the current PRF mechanism for engine combustion simulations.

the current mechanism. Detailed information about these engines are shown in Table 5. Table 5. Engine Specifications and Operating Conditions ERC GM 1.9 L

engine type

Sandia GM 1.9 L

CAT 3401

YC6J

137.2 × 165.1 2.44 L 16.1:1

105 × 125 1.081 L 15.8:1 re-entrant

bore × stroke [mm] displacement compression ratio piston bowl

90.4 × 82

90.4 × 82

0.477 L 17.4:1

0.477 L 16.4:1

re-entrant

re-entrant

injector type engine speed [r/min] intake pressure [bar] intake temperature [K] fuel mass [mg] injection pressure [bar] SOI timing [ATDC] equivalence ratio EGR rate

port injection 1500

CR DI 1500

Mexican hat CR DI 700

1.1

1.5

1.0

1.35

303, 333, 363 372

381, 389

303

− −

8.8 860

− −

40.0 1200

Port injection

−23.3

−274.5

−8.0

0.26, 0.3

0.3

0.23, 0.29

−

0%

10% O2, 81% N2 9% CO2 PRF25

0%

30%, 50%

gasoline

diesel gasoline

fuel

n-heptane iso-octane

CR DI 1500

The Kiva CFD code was used to provide the CFD framework.20 The related submodels for the current simulation are shown in Table 6. The SpeedChem code19 was used for the Table 6. Submodels Used in the KIVA Code for Engine Combustion Simulations phenomenon

model

reference

spray breakup evaporation turbulence combustion droplet collision near nozzle flow

KH-RT instability discrete multicomponent fuel (DMC) renormalized k−ε model sparse analytical Jacobian (SpeedChem) radius of influence (ROI) model gas-jet model

48, 49 50 51 19 52 53

chemistry calculation, and typical running timings were around 3 h for the HCCI simulations and 7.5 h for the DI spray combustion simulations with two processors. Figure 10 shows the computational grids for the engine combustion simulations. Forty five degree sector meshes were applied for all the engines to improve computational efficiency. The cell numbers of the ERC-GM engine, Sandia-GM engine,

Figure 10. Computational grids for HCCI, PPCI, and DI combustion simulations. H


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Figure 11. Comparisons between simulations and experiments of CAT engine HCCI combustion. Experimental data are from refs 44 and 45.

Figure 12. Comparisons between simulations and experiments for ERC-GM PRF HCCI combustion. Experimental data are from ref 46.

be further extended for PAH and soot predictions. The results can be summarized as follows: (1) A reduced PRF mechanism with 73 species and 296 reactions was developed based on the detailed LLNL mechanism. The reaction pathways of the present reduced nC7H16 and i-C8H18 mechanism follow the general reaction pathways of Curran et al.4,5 The major reaction pathways of the detailed mechanism were mostly retained in the reduced mechanism, which ensures the accuracy and the ability of the mechanism to be extended to other fuels as well as the computational efficiency of the reduced mechanism. The reaction rates of two key reactions involved in the H2−O2 system were also evaluated and updated. (2) The reduced mechanism predicts well the ignition timings, laminar flame speeds, and important species profiles under various validation conditions, including the important intermediate species that are related to PAH and soot formation. This means that the current mechanism can be used as a base mechanism for the prediction of PAH formation when a PAH mechanism is incorporated into the mechanism. (3) The mechanism was further validated with available HCCI and PPCI combustion data and also diesel/gasoline direct-injection combustion experimental data. The mechanism predicted combustion of iso-octane and PRF fuels in HCCI combustion accurately under different operating conditions. The major combustion characteristics can be predicted well by the present mechanism in both PPCI and diesel/gasoline DI spray combustion cases over a wide range of EGR rates. (4) The overall results show that the current mechanism can provide reliable predictions for both diesel and gasoline engine combustion simulations. The predictive ability of important species related to PAH formation indicates that the mechanism

Figure 13. Comparisons between simulations and experiments for Sandia GM PRF PPCI combustion. Experimental data are from ref 47.

4. CONCLUSIONS A reduced PRF mechanism was developed for combustion simulations of PRF and diesel/gasoline surrogate fuels based on detailed LLNL mechanisms. The reduced PRF mechanism consists of 73 species and 296 reactions. Important reaction pathways and reactions in the reduced mechanism were identified and are discussed and updated in the study. In particular, the reaction rates of two reactions, HO2 + OH = HO2 + O2 and HO2 + HO2 = H2O2 + O2, in the hydrogen submechanism were updated. Extensive validations were conducted to verify the predictive ability of the proposed reduced mechanism. The overall results suggest that the proposed mechanism can provide reliable predictions for PRF and diesel/gasoline surrogate combustion simulations and can I


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Figure 14. Comparisons between simulations and experiments for diesel/gasoline mixture, direct-injection spray combustion cases. (10) Sankaran, R.; Hawkes, E. R.; Chen, J. H.; Lu, T.; Law, C. K. Structure of a spatially developing turbulent lean methane−air Bunsen flame. Proc. Combust. Inst. 2007, 31 (1), 1291−1298. (11) Turanyi, T.; Tomlin, A. S.; Pilling, M. J. On the error of the quasi-steady-state approximation. J. Phys. Chem. 1993, 97 (1), 163− 172. (12) Lu, T.; Law, C. K. Strategies for mechanism reduction for large hydrocarbons: n-Heptane. Combust. Flame 2008, 154 (1−2), 153− 163. (13) Tanaka, S.; Ayala, F.; Keck, J. C. A reduced chemical kinetic model for HCCI combustion of primary reference fuels in a rapid compression machine. Combust. Flame 2003, 133 (4), 467−481. (14) Liu, Y.-D.; Jia, M.; Xie, M.-Z.; Pang, B. Enhancement on a Skeletal Kinetic Model for Primary Reference Fuel Oxidation by Using a Semidecoupling Methodology. Energy Fuels 2012, 26 (12), 7069− 7083. (15) Klippenstein, S. J.; Harding, L. B.; Davis, M. J.; Tomlin, A. S.; Skodje, R. T. Uncertainty driven theoretical kinetics studies for CH3OH ignition: HO2 + CH3OH and O2 + CH3OH. Proc. Combust. Inst. 2011, 33 (1), 351−357. (16) Tsurushima, T. A new skeletal PRF kinetic model for HCCI combustion. Proc. Combust. Inst. 2009, 32 (2), 2835−2841. (17) Luong, M. B.; Luo, Z.; Lu, T.; Chung, S. H.; Yoo, C. S. Direct numerical simulations of the ignition of lean primary reference fuel/air mixtures with temperature inhomogeneities. Combust. Flame 2013, 160 (10), 2038−2047. (18) Shi, Y.; Green, W. H., Jr.; Wong, H.-W.; Oluwole, O. O. Redesigning combustion modeling algorithms for the Graphics Processing Unit (GPU): Chemical kinetic rate evaluation and ordinary differential equation integration. Combust. Flame 2011, 158 (5), 836− 847. (19) Perini, F.; Galligani, E.; Reitz, R. D. An Analytical Jacobian Approach to Sparse Reaction Kinetics for Computationally Efficient Combustion Modeling with Large Reaction Mechanisms. Energy Fuels 2012, 26 (8), 4804−4822. (20) Amsden, A. Kiva-3v, Release 2, Improvements to Kiva-3v, 1999, LA-UR-99-915. (21) Kee, R.; Rupley, F.; Miller, J. CHEMKIN-II: A Fortran Chemical Kinetics Package for the Analysis of Gas-Phase Chemical Kinetics; Sandia Natl. Lab. [Technical Report] SAND, 1989 (accessed July 25, 2012). (22) Mehl, M.; Pitz, W. J.; Westbrook, C. K.; Curran, H. J. Kinetic modeling of gasoline surrogate components and mixtures under engine conditions. Proc. Combust. Inst. 2011, 33 (1), 193−200. (23) Pepiot-Desjardins, P.; Pitsch, H. An efficient error-propagationbased reduction method for large chemical kinetic mechanisms. Combust. Flame 2008, 154 (1−2), 67−81. (24) Shi, Y.; Ge, H.-W.; Brakora, J. L.; Reitz, R. D. Automatic Chemistry Mechanism Reduction of Hydrocarbon Fuels for HCCI Engines Based on DRGEP and PCA Methods with Error Control. Energy Fuels 2010, 24 (3), 1646−1654.

is also suitable to predict PAH formation by introducing a PAH mechanism into this base mechanism.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge financial support provided by the Princeton Combustion Energy Frontier Research Center and the Sandia National Laboratories. The authors are also thankful for support from Reaction Design for the chemistry simulations with the CHEMKIN Pro software.

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REFERENCES

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Development of a Reduced Primary Reference Fuel Mechanism for

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