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Cite This: Energy Fuels XXXX, XXX, XXX−XXX

A Shock Tube Experimental and Modeling Study of Multicomponent Gasoline Surrogates Diluted with Exhaust Gas Recirculation Hua Li, Liang Yu, Shuzhou Sun, Sixu Wang, Xingcai Lu,* and Zhen Huang* Key Laboratory for Power Machinery of M.O.E., Shanghai Jiao Tong University, Shanghai 200240, P. R. China S Supporting Information *

ABSTRACT: The ignition delay for pure cyclohexane and two quaternary gasoline surrogate fuels CTRF1 (isooctane/nheptane/toluene/cyclohexane, 37.070/11/40.076/11.854 by mole fraction), CTRF2 (isooctane/n-heptane/toluene/cyclohexane, 30.538/11/43.077/15.385 by mole fraction) with the same research octane number of 95 (RON = 95) in air is measured under lean, stoichiometric, and rich conditions behind reflected shock waves, at temperatures of 1027 K−1400 K, and pressures of 10, 15, and 19 bar/20 bar. To analyze the effects of exhaust gas recirculation upon ignition, CTRF1/air mixtures are diluted with CO2 to simulate different exhaust gas recirculation (EGR) loadings (0, 20%, 40%, and 60%). The experimental data are compared to the predictions calculated by a detailed chemical kinetic mechanism with 526 species and 2763 reactions generated in this work, which is also validated by the autoignition characteristics of pure cyclohexane, iso-octane, n-heptane, toluene, and their binary and ternary mixtures. The simulation results of the mechanism are in good agreement with the experimental measurements, and both the experimental and kinetic modeling data illustrate a negative correlation between the ignition delay of CTRF1/CTRF2 and the pressure, temperature, and equivalence ratio, and a clear rise of the ignition delay for CTRF1 with increased EGR loadings is also showed. Moreover, on the basis of the detailed kinetic model, the reaction pathway, rate of production analysis, and sensitivity analysis are also performed to clarify the influence of EGR on the ignition delay of CTRF1, which indicates the predominate role of thermal and dilution effects that CO2 has on the ignition, while the chemical effects are proven negligible over the range of experimental conditions.

1. INTRODUCTION The overuse of fossil fuels and the increasing stringent emission regulations call for the optimization of the internal combustion engine (ICE). Since the characteristics of fuels have a significant effect on the combustion process of ICE, there has been a continual interest in researching the autoignition properties of gasoline, which is mostly used as a conventional fuel. Numerical simulations may deepen the understanding of combustion process in engines, diminish the research cost, and guide the development of advanced combustion modes. Thus, it is extremely urgent to develop the chemical kinetics for relative fuels. However, for a petroleum fuel such as gasoline, which is composed of a full spectrum of hydrocarbons that vary with location and refinery process, it is difficult to explore its fundamental combustion and chemical kinetics directly. Gasoline surrogates, comprise limited hydrocarbons belonging to different chemical families, therefore they are widely adopted in both experimental and modeling research to replicate the combustion properties of real gasoline, and the proportion of each component is calculated according to a specific methodology with respect to the nature of target fuels. Originally, various research groups focused on iso-octane, nheptane, and toluene due to their representative positions. Later, in an effort to reproduce more characteristics of practical gasoline, such as H/C ratio, octane rating, octane sensitivity, and molar mass, more complex surrogates with quite a number of compositions to account for additional chemical families have been proposed, ranging from binary surrogate PRF (primary references fuels) and ternary surrogate TRF (toluene reference fuels) to multicomponent mixtures.1,2 Since naph© XXXX American Chemical Society

thene occupies a significant role in conventional fuels (up to 10% of gasoline), and influences the growth of benzene and polycyclic aromatic hydrocarbons (PAH) by its oxidation process at low temperature range, cyclohexane (CHX) is investigated grossly as a component in gasoline surrogate in light of its simplest structure. Lemaire et al.3 found the obvious NTC behavior of CHX through the experimental data measured by a rapid compression machine (RCM) at pressures of 0.7−1.4 MPa and temperatures of 600−900 K. Tian et al.4,5 validated the prediction ability of three models of cyclohexane with the shock tube (ST) data at temperatures of 1075−1750 K, at pressures of 1.1, 5, and 16 atm. Kiefer et al.6 studied the decomposition of cyclohexane bathed in Kr in a shock tube over 1300−2000 K and 25−200 Torr. Meanwhile, Hong et al.7 made comparisons of ignition delay of cyclohexane, methylcyclohexane, and n-butylcyclohexane, and Wang et al.8 detected and quantified the pyrolysis and oxidation products of cyclohexane at 40 mbar in a plug flow reactor. Different from the majority of ST tests in which cyclohexane was diluted with Ar, the work of Daley et al.9 involved the autoignition of cyclohexane/air mixtures at temperatures of 847−1379 K, pressures of 11−61 atm, and equivalence ratios of 1.0, 0.5, and 0.25 when the bath gas was N2, providing a database for model validation under engine conditions. Moreover, the chemical kinetics of cyclohexane keeps developing in recent years, especially for its oxidation process. Received: April 11, 2017 Revised: January 23, 2018 Published: February 6, 2018 A

DOI: 10.1021/acs.energyfuels.7b01028 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Buda et al.10 constructed a detailed low-temperature mechanism involving 513 species and 2446 reactions for cyclohexane, which may reproduce the ignition delay of cyclohexane under 700 K in an RCM, but overpredict the NTC behavior for the lack of addition reactions of radicals to oxygen molecules. Additionally, Serinyel et al.11 revised this model using EXGAS and validated it with the mole fraction profiles of 34 products produced in the oxidation procedure of cyclohexane. Besides, Sirjean et al.12,13 conducted the ST experiments of cyclohexane/air mixtures under the conditions of 1230−1840 K and 7.3−9.5 atm and built a high-temperature oxidation mechanism model for cyclohexane (372 species and 1629 reactions), which was further updated with the isomerization reactions between cyclohexylperoxy and cyclohexylhydroperoxy radicals as well as the decomposition reactions of cyclohexylhydroperoxy radicals into cyclic ethers and OH. On the basis of the development of gasoline surrogates, EGR (exhaust gas recirculation) is studied extensively owing to its effects on reducing the NOx formation and its favorable attributes to control HCCI combustion. There are numerous studies on the application of EGR in engines regardless of experiments or numerical simulation. Gu et al.14studied the emission characteristics of gasoline/n-butanol mixtures in a three-cylinder port fuel injection spark ignition engine at different EGR rates. It was found that the EGR dilution increased the emission of CO and HC slightly while the NOx and particle number concentration decreased significantly. Huang et al.15 found that the effects of EGR were weak on the regular emission and maximum pressure rising ratio when the EGR rate was less than 25% based on the combustion of diesel/gasoline/n-butanol in a four-cylinder compression ignition engine. In addition to traditional fossil fuels, the effects of EGR on surrogates and additives have also been extensively studied by means of numerical simulations. Machrafi et al.16 and Vourliotakis et al.17 used the 0dimensional single-zone engine model to simulate the combustion of PRF95, TRF, and PRF80 at conditions similar to HCCI operation with the simulated EGR of N2 and CO2 separately. Besides, Zheng et al.18 established a skeletal kinetic model for TRF-NO, and regarded NO as exhaust gas to investigate the variety of the ignition delay with different EGR rates, similar to the research of Andrae et al.19 The control of the new combustion modes such as LTC and HCCI also require accurate expressions of flame of fuels with EGR loading. Middleton et al.20 adopted the transient 1dimension laminar flame model to study the effects of EGR on the flame propagation of iso-octane/air in which the synthetic EGR consisted of 20% CO2 and 80% N2. The results identified the decrease of burning velocity attributed to the increase of the EGR rate. Hu et al.21 investigated the laminar flame speed of CH4/O2 diluted with CO2, and found that the addition of CO2 resulted in the increase of flame thickness. Moreover, Chan et al.22 and Zahedi et al.23 used a flat-flame burner to study the flame of CH4/O2 and NO emission with the dilution of CO2, while Tang et al.24 studied the correlation of the laminar flame velocity and the dilution rate of N2 through the combustion of propane/air in spherically expanding flames. Besides, Kashif et al.25 utilized a coflow burner to research the sooting propensities of two binary gasoline surrogate n-heptane/ toluene and iso-octane/toluene with the simulated EGR of CO2 in diffusion flames. Compared with the studies of EGR on engines, the research of the EGR effects on the properties of autoignition is relatively

scarce. Gauthier et al.26 measured the ignition delay of gasoline RD387 and two surrogates with simulated EGR represented by CO 2 /H 2 O/O 2 /N 2 under HCCI conditions in an ST. Vandersickel et al.27 measured the ignition delay of n-heptane and two practical kerosene-like fuels in an ST at EGR ratios of 0, 30%, and 50%, and the results revealed that the larger dilution put off the ignition of n-heptane and flattened the negative temperature coefficient (NTC) region in the medium temperature range, in which the EGR was simulated by N2. Zeng et al.28 conducted ST experiments to research the ignition of methane diluted with 0, 20%, and 50% N2/CO2 and indicated the stronger inhibitory effect of CO2, while Ramalingam et al.29 studied the dilution effects of Ar/N2 on the ignition of methane in an RCM experiments at EGR ratios of 0, 10%, and 30%. Throughout the research of neat cyclohexane, the overwhelming majority of ignition experiments are conducted with argon dilution, which is not in accordance with the real operation conditions of an ICE. In this work, the ignition properties of cyclohexane/air mixtures are detected over a wide range of conditions in a high-pressure ST, as compared with the predictions of different kinetic models. In consideration of the lack of research on gasoline surrogates including cyclohexane, two quaternary surrogates comprising iso-octane/n-heptane/ toluene/cyclohexane named CTRF1 and CTRF2 are constructed according to the key characteristic of a target gasoline with a RON = 95, and the ignition delay of the two surrogates is measured in the ST at different pressures, temperature ranges, and EGR loadings. Furthermore, a chemical kinetic model involving 526 species and 2763 reactions is built for the quaternary surrogates, and validated with each neat component and their binary and ternary mixtures under a variety of conditions. Also, a series of reaction fluxes, sensitivity analyses, and concentration variations are analyzed based on this model aiming at discussing the impediment that EGR had on ignition.

2. EXPERIMENTAL METHODOLOGY All of the ignition delay experiments were performed in an unheated high-pressure shock tube facility with 90 mm inner diameter, which was described and tested in detail in references.30−32 The shock waves were produced by the rupture of diaphragm, and the arrival of the incident and reflected waves were detected by six piezo-electric pressure transducers (PCB113B26) positioned axially along the driven section, whereas the OH* emission was detected by a photomultiplier (Hamamatsu, R928). The purities of all gases (N2, O2, He, CO2) used in this study were 99.999%, toluene, isooctane, and cyclohexane were 99.5%, n-heptane was 99.9%. To ensure sufficient blending, all fuels were mixed with air (N2/O2 = 3.76:1) in a stainless-steel tank for at least 12 h at room temperature before the experiments at every operation condition, and the partial pressures of all components should be kept less than their saturated vapor pressure individually at the corresponding temperature during the entire experiment process. To guarantee the uniformity of the experimental method, in correspondence with references,7,33,34 the ignition delay in this study is defined as the time interval between the time that the reflected wave arrives at the last pressure transducer and the intercept of the maximum slope of the OH* emission profile back to the baseline, as is shown in Figure 1. The gasoline surrogates are constructed in two main steps. First is the selection and confirmation of components. Not only the constituents of the target fuel but also the current knowledge of their chemical kinetics should be considered. It means that the surrogate is composed of typical species belonging to different chemical families in the light of research goals. Second, the proportion of each compound should be calculated precisely, which is necessary to choose appropriate methodology with closed equations and to arrange B


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ignition delay of neat cyclohexane, CTRF1, and CTRF2 was measured in an ST behind the reflected shock at temperatures of 1027−1400 K, pressures of 10−20 bar, equivalence ratios of 0.5−2.0, and the total blending ratios of the surrogate/air mixtures under all operation conditions in the ST are listed in Table 2. In addition, a regression analysis is carried out with the experimental data of cyclohexane/air mixtures, fitted by an Arrhenius formula (Formula 1), τ = AP−n exp(Ea/RT), where R represents the universal gas constant of 1.986 × 10−3 kcal mol−1 K−1, Ea is the overall activation energy in kilocalories, T is the temperature in kelvin, P denotes the reflected shock pressure in bar, A is the pre-exponential factor, and τ is the ignition delay time in seconds. For CHX, Φ = 1.0:

τ = 2.825 × 10−8P−1.064 exp(27.33/(RT )) Figure 1. Definition of the ignition delay.

(1) 9

Moreover, the experimental data presented by Daley et al. is cited to verify the reliability of the ST. Daley et al. investigated the ignition delay of cyclohexane/air mixtures at equivalence ratios of 1.0, 0.5, and 0.25, pressures of 11−61 atm, and temperatures of 847 to 1379 K in a high-pressure shock tube facility. Herein, a comparison is made by scaling the data of this study and that of Daley et al. to 15 bar by τ ∝ P−1.064 and τ ∝ P−1.1 independently, elucidated in Figure 2 with

the priority of constraints. In this study, isooctane/n-heptane/toluene/ cyclohexane were selected to represent the branched-alkanes/linearalkanes/aromatics/cycloalkanes due to the classification of commercial gasoline composition. Employing the method proposed by Pera et al.,35 RON = 95, H/C = 1.801, and octane sensitivity = 8 were selected as the major constraints in terms of the gasoline properties. Combined with the restriction of GB 18352.5−2013 (aromatics vol % ≤ 35%), two quaternary gasoline surrogate fuels CTRF1 (isooctane/n-heptane/ toluene/cyclohexane, 37.070/11/40.076/11.854 by mole fraction), and CTRF2 (isooctane/n-heptane/toluene/cyclohexane, 30.538/11/ 43.077/15.385 by mole fraction) were built, and the properties of neat alkanes and the quaternary surrogates are listed in Table 1. The

Table 1. Properties of Neat Hydrocarbons and the Quaternary Surrogates at 298 K

RON MON H/C ratio molar weight (g/mol)

isooctane

nheptane

toluene

cyclohexane

CTRFa

CTRFb

100 100 18/8 114.2

0 0 16/7 100.2

120 103.5 8/7 92.1

83 77.2 12/6 84.2

95 87.7 1.801 100.3

95 87 1.7547 98.5

Figure 2. Comparison of the ignition delay measurements of cyclohexane/air mixtures in this study scaled to 15 bar as P−1.064 with the experimental data of Daley et al.9 scaled to 15 bar as P−1.1 at the same condition.

a

The properties of neat hydrocarbons are from ref 35. bH/C ratio and molar weight of surrogates are computed according to the equations in ref 35.

Table 2. Composition of the Mixtures in This Study at Different Experimental Conditions (mol %) Φ CHX

CTRF1

CTRF2

1 1 1 0.5 1.5 1 1 1 0.5 2 1 1 1 1 1 1 0.5 2

isooctane

0.7260 0.7260 0.7260 0.3666 1.4240 0.5808 0.4356 0.2904 0.6111 0.6111 0.6111 0.3086 1.1982

n-heptane

0.2154 0.2154 0.2154 0.1088 0.4226 0.1723 0.1293 0.0862 0.2201 0.2201 0.2201 0.1112 0.4316

toluene

cyclohexane

0.7848 0.7848 0.7848 0.3963 1.5395 0.6279 0.4709 0.3139 0.8620 0.8620 0.8620 0.4353 1.6901

2.2810 2.2810 2.2810 1.1537 3.3829 0.2321 0.2321 0.2321 0.1172 0.4554 0.1857 0.1393 0.0929 0.3079 0.3079 0.3079 0.1555 0.6036 C

CO2

20 40 60

O2

N2

P (bar)

20.5292 20.5292 20.5292 20.7660 20.2977 20.5970 20.5970 20.5970 20.8007 20.2014 16.4776 12.3582 8.2388 20.5880 20.5880 20.5880 20.7961 20.1841

77.1898 77.1898 77.1898 78.0803 76.3194 77.4447 77.4447 77.4447 78.2105 75.9571 61.9557 46.4668 30.9779 77.4110 77.4110 77.4110 78.1933 75.8924

10 15 19 19 19 10 15 20 20 20 20 20 20 10 15 20 20 20


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Figure 3. Comparison of the ignition delay of CHX between the experimental data and simulated results based on different models at P = 10 bar/15 bar/19 bar (solid circles, experimental data measured in this study; lines, simulation results by Jet-Surf 2.0;47 dash lines, simulation results by the mechanism of Silke et al.48).

Table 3. Modifications of the Reactions no.

reaction

A

n

Ea (cal/mol)

1908 1940 2114 2129 2130 2241 2484 2655 2678 2734

C2H5+C2H5(+M) = C4H10(+M) C2H4+C2H5(+M) = pC4H9(+M) C4H6+H = C2H4+C2H3 C4H6+OH = nC4H5+H2O C4H6+OH = iC4H5+H2O C2H4+CH3 = nC3H7 C2H4+O = CH3+HCO CH3+HO2 = CH3O+OH CH2O+OH = HCO+H2O OH+OH(+M) = H2O2(+M)

1.88 × 1014 1.50 × 1011 5.45 × 1030 6.20 × 1006 3.10 × 1006 3.300 × 1011 1.920 × 1007 1.34 × 1013 3.43 × 1009 1.110 × 1014

−0.5 0 −4.51 2.0 2.0 0 1.830 0 1.180 −0.37

0 7300 21877 3430 430 7700 220 0 −447 0

scatters. Meanwhile, the maximum overall uncertainty in ignition delay of approximately ±15% was indicated with an error bar in this figure owing to the combination of boundary layer effects and the uncertainty of the temperature measurements.4,36,37 It can be concluded that both of the two data sets of the ignition delay show analogous tendency with temperature, and no NTC behavior can be observed under this condition, corresponding to the temperature and pressure dependence in the formula mentioned above. However, the magnitude of the ignition delay in this study is slightly larger than that of Daley et al., and the discrepancy decreases around 1000 K, which may be ascribed to the uncertainty in measurement of the two ST facilities. Additionally, the postshock conditions (T5/P5) are resolved by Gaseq,38 and the regression analysis uncertainty (R-square) is 0.9818 based on the least-squares method in this case, while the determination of these by Daley et al. was not given in their study, which may play a part in the inconsistency between experimental data.

predicting the combustion property of each elementary substance after combination. Aimed at developing the chemical kinetics of the quaternary gasoline surrogate fuels containing isooctane, n-heptane, toluene, and cyclohexane, the generation of the whole multicomponent model may be divided into three parts namely primary reference fuels (PRF), toluene, and cyclohexane, simplifying the scope of the model as far as possible on the premise of sufficient precision. Yuan et al.42,43 developed a new chemical model for toluene including 272 species and 1698 reactions by adding the submechanism of benzene and 1,3cyclopentadiene to the C0−C4 core mechanism updated by Cai et al.,44 which was built based on USC Mech II. Benefiting from its maturity covering pyrolysis and oxidation, this toluene model is selected as the major chemical kinetics to blend. Besides, the submechanism of PRF and cross-reactions of primary fuel radicals presented by the skeletal mechanism (323 species) of Mehl et al.45,46 are adopted to shrink the scale and represent the NTC behavior at a lower temperature range more exactly. As mentioned above, the chemical model of cyclohexane has drawn more and more attention because of its simplest structure in naphthene. Figure 3 compares the prediction ability of the ignition delay of Jet-Surf 2.047 and Silke et al.48 for a stoichiometric cyclohexane/air mixture at pressures of 10−19 bar. It is obvious that the results calculated by Jet-surf 2.0 is closer to the experimental data measured in this study (solid circles), while Silke overpredicts the ignition delay below 1030 K and under-predicts it as the temperature rises. Therefore, the submechanism including the low temperature chemistry of cyclohexane in Jet-Surf 2.0 is chosen to blend to the major model, and a chemical kinetic model

3. RESULTS AND DISCUSSION 3.1. Chemical Mechanism Development and Validation. The mainstream methodology39−41 to build chemical kinetics of multicomponents was adopted in this case. Generally, one can select the suitable submechanism for each composition based on the experiment operation conditions first, and specify the major model out of them against research goals before blending them together. Then, add all submechanisms to the major one, in which the species should be distinguished and unified according to the thermodynamic data. After that, blend all the thermodynamic database together followed by the deletion of duplicate reactions in the submechanisms with CHEMKIN interpreter. Finally, the rate constants of some key reactions may be adjusted validly to improve the prediction ability. It is noteworthy that the new multicomponent chemical model may maintain its efficiency in D


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Figure 4. Experimental and modeling results of the ignition delay for cyclohexane/air mixture at multiple conditions (symbols, experimental data in this study and Lemaire;3 lines, simulation results by the new model proposed in this study).

ratios below 1200 K, similar to the conclusion of Daley et al. but opposite to the results of a previous study in which there is dilution with a high concentration of Ar. In the meantime, the new multicomponent model may reproduce the experimental data at 10, 15, and 19 bar identically, especially for the rich CHX/air mixture (Φ = 1.5), which indicates that the prediction ability of cyclohexane submechanism at high temperatures is not influenced by combination. Simultaneously, the low temperature chemistry of cyclohexane is also proven through the comparison of prediction of ignition delay with experimental data measured in a rapid compression machine by Lemaire et al.,3 which is presented in Figure 4 (c). It is concluded that the nascent model shows good agreement in the temperature range of 667−909 K, even the NTC (negative temperature coefficient) behavior is reproduced. Moreover, a great deal of experimental data for iso-octane, nheptane, and toluene from the literature is used to verify the prediction ability of the model proposed, which is shown in Figures S1−S3 in the Supporting Information. The simulation results agree well with the measurements over a wide range of temperatures, pressures, and equivalence ratios, and the shift of the NTC region with the variation of pressures for n-heptane is also captured. In addition, the ignition delay of binary and ternary gasoline surrogates blended with iso-octane, n-heptane, and toluene is also modeled by the nascent model, and the comparison with the experimental data shows good agreement, which is illustrated in Figures S4−S6 in the Supporting Information. Recently, Sarathy et al.49 developed a chemical kinetic model comprising various hydrocarbon submodels with 2315 species

involving 526 species and 2763 reactions is completely built for the quaternary surrogates. The ignition delay measured by the shock tube can provide excellent validation targets for refinement of kinetic mechanisms at the temperatures of 700−1300 K and pressures of 10− 60 atm. The main purpose of the model constructed in this study is to capture the autoignition of gasoline surrogates at engine-like operation conditions precisely. Hence, the ignition delay of CTRF1 and CTRF2 is measured in the shock tube, and the experimental data of each compound and their binary and ternary mixtures in the literature is adopted for validation. The constant volume closed homogeneous 0-dimension model in CHEMKIN is used, while the pressure derivation named dp/ dt is not taken into consideration due to an average pressure rise of less than 2% at the middle to high temperature range in all experiment conditions, and a specific volume profile is used to simulate the ignition delay for the nonideal measurement with longer test time (ignition delay > 1 ms). The measurement and analyzed data are expressed in scatters and lines individually in all figures. The blended chemical kinetic model for multicomponent fuels may simultaneously predict the combustion features of each constituent. To enhance the prediction accuracy, the rate constants of some key reactions listed in Table 3 are replaced with the original ones in USC-Mech II through repetitive computation. In this study, the simulated results are first compared to the ignition delay of cyclohexane/air mixtures at pressures of 10, 15, and 19 bar, equivalence ratios of 0.5, 1.0, and 1.5, shown in Figure 4a,b. As expected, the ignition delay of CHX depicts a gradual decline with the rise of equivalence E


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Figure 5. Comparison of the experimental and analyzed data of the ignition delay for CTRF1 (symbols, experimental data in this study; lines, simulation results by the model proposed in this case; dash and short dash dot, simulation results by the model of Sarathy et al.49).

Figure 6. Comparison of the experimental and analyzed data of the ignition delay for CTRF2 (symbols, experimental data in this study; lines, simulation results by the model proposed in this case; short dash and dot, simulation results by the model of Sarathy et al.49).

chemistry of the ignition process under same conditions for CTRF1 and CTRF2 with the same RON may not be influenced strongly by the change of concentration of each component, for the corresponding coefficients in Formulas 2 and 3 show slight divergence. For CTRF1, Φ = 1.0:

and 10079 reactions, aiming at simulating the ignition of surrogate fuels. Figure 5 and Figure 6 demonstrate the performance of the two newly built models in comparison with the experimental measurements of CTRF1 and CTRF2 in this study at pressures of 10, 15, and 20 bar over a wide range of equivalence ratios. It can be deduced from Figures 5a and 6a that CTRF1 and CTRF2 ignite earlier with the increase of the temperature at 1000−1300 K, which is the same as the properties of each component in the surrogate. Moreover, there exists a negative correlation between the ignition delay and the pressure for both CTRF1 and CTRF2, which indicates little distinction of overall activation energies when the pressure climbs from 10 to 20 bar. For stoichiometric mixtures at all pressures, the data are well predicted by the two models below 1180 K, while Sarathy et al.49 overpredicts the ignition delay of both CTRF1 and CTRF2 with ascending temperature. Figures 5b and 6b present the ignition delay of CTRF1 and CTRF2 at different equivalence ratios, and the discrepancy of the activation energy among different equivalence ratios is apparent. Over the whole experimental conditions, the simulated data of both Sarathy et al.49 and the nascent model (SJTU) in this work show good predictions no matter the magnitude or tendency, but the ignition delay simulated by Sarathy et al.49 is larger than that of the SJTU out of the temperature range of 1065−1210 K. Furthermore, a regression analysis expressed in Arrhenius format is carried out with the experimental data of CTRF1 and CTRF2 over three different pressures at Φ = 1.0. It can be concluded that the combustion

τ = 2.081 × 10−9 × P−1.079 exp(34.19/(RT ))

(2)

For CTRF2, Φ = 1.0: τ = 2.353 × 10−9 × P−0.9324 exp(33.12/(RT ))

(3)

3.2. The Effects of EGR on the Ignition Delay of CTRF1. More and more researchers focus on the exhaust gas recirculation (EGR) for its effects on reducing the NOx emission. In this case, CO2 is selected to simulate EGR to investigate the influence that different EGR loadings (0, 20%, 40%, and 60%) had on the autoignition of CTRF1 under conditions of Φ = 1.0 and P = 20 bar. The EGR rate in this study is defined such that a mixture with X% EGR corresponds to a mixture of (100 − X) mol % of the fuel/air mixture and X mol % of CO2. Figure 7 presents the comparison of the measurements of the ignition delay for CTRF1 in a shock tube with the analyzed data calculated by the model proposed in this study at 20 bar, together with the modeled results obtained by Sarathy et al.49 It is clear to see that the ignition delay shows a gradual decline with the decrease of EGR rate from 60% to 0 under this condition, and the simulated results using the model F


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It is generally believed that EGR may affect the ignition and combustion of fuels mainly from three aspects: thermal effects, dilution effects, and chemical effects. The thermal effects refer to the polyatomic molecules in the exhaust with high specific heat capacity such as H2O and CO2, which may reduce the temperature of the unburned mixture and delay the ignition; the dilution effects result from the drop of O2 concentration by the occupation of exhaust gas in the charge; and the reactions involved in exhaust gas or influenced by the third body collision may promote the generation of radicals such as OH, leading to the chemical effects. However, there are divergent opinions about the chemical effects that exhaust gas has. Zeng et al.28 studied the autoignition properties of methane in an ST under dilution conditions of CO2 and N2 and found that the activity of the oxygen-rich mixture was not changed when the dilution rate is 20%. Donohoe et al.50 researched the effects of steam on hydrogen, syngas, and natural gas, and the results showed that the thermal properties were changed dramatically while the chemical effects were not obvious. On the contrary, Di et al.51 proved the ‘promotion chemical effects’ of CO2 on the ignition of iso-octane especially at temperatures above 850 K using an RCM, and Desantes et al.52 believed that the addition of H2O advanced the ignition of iso-octane at intermediate temperatures, but the chemical effects of CO2 were negligible. So, it is necessary to analyze the effect principle of CO2 addition on the ignition of CTRF1 with increasing EGR rate. For further investigation of how the EGR affects the autoignition of CTRF1, a sequence of reaction pathways is established at 1000 K, with the pressure of 20 bar and equivalent ratio of 1.0 when the consumption of oxygen is 20%. Figure 9 presents the reaction flows of each component in CTRF1 with different EGR loadings, where the numbers indicated the contribution of each path in ascending order of the EGR rates (red, EGR0; black, EGR20%; pink, EGR40%; blue, EGR 60%). In the atmosphere of oxidation, the major consumption path of each compound and their products are the H-abstraction reactions by free radicals to form the radical pool. For toluene, the H loss by OH attack to form benzyl (A1CH2) and methylphenyl (C6H4CH3) is strengthened with the increase of the EGR rate. And cyclohexane is 100% converted to cyclohexyl radical (cC6H11) followed by the isomerization to produce hex-6-en-1-yl (PXC6H11) radical whose conversion rate decreases from 72.7% to 58.1% when the EGR rate increases from 0 to 60%. The reaction pathways of iso-octane and n-heptane are also influenced by the rise of the EGR loading; the channels to produce NEOC5H11, PXC5H11, pC4H9, and C6H13-1 totally disappear when the EGR rates climb to 60%. It is noteworthy that the dominant decomposition channels of each component in CTRF1 remain the same with the variation of EGR rates, despite the insignificant change of the conversation ratio of some paths. And this may be caused by the decrease in the overall activity of the unburned mixture diluted by CO2. Although there is no significant difference of the reaction flux except the change of contribution, the overall activity of the system may be affected by the addition of CO2. To identify the chemical effects of exhaust gases on the ignition of CTRF1, the normalized first-order sensitivity coefficients of the OH radical with respect to pre-exponentials for the stoichiometric CTRF1/ air mixture with different EGR loadings are calculated through CHEMKIN under conditions of 20 bar, 1000 K, and the top 10 reactions at 0, 20%, 40%, and 60% EGR were presented in Figure 10 panels a, b, c, and d, respectively. The negative

Figure 7. Comparison of the experimental and analyzed data of the ignition delay for CTRF1 at different EGR loadings (symbols, experimental data in this study; lines, simulation results by the model proposed in this case; dash, simulation results by the model of Sarathy et al.49).

proposed in this study match quite well with the measurements of ignition delay with all four EGR loadings, while Sarathy et al.49 overpredicts the ignition when the temperature is above 1250 K. Figure 8 shows the simulated results of the ignition

Figure 8. Variation of the ignition delay for CTRF1 calculated with the new built model with different EGR rates at 800, 1000, and 1500 K.

delay for CTRF1 with different EGR loadings at T = 800, 1000, and 1500 K. Similar to the dependence on temperature, the ignition delay of stoichiometric CTRF1/air mixture grows approximately exponentially with the increase of EGR rate at a specific temperature. As shown in Figure 8, the ignition delay is 10.994 ms with no EGR, but climbs to 16.326 ms as the EGR rate rises to 20%, and the total rate of ascent is almost 430% when the EGR rate rises from 0 to 60% at T = 800 K. Formula 4 in Arrhenius format is fitted by the regression analysis (Rsquare = 0.9925) of ignition delay measurements at all EGR conditions. The global activation energy alters little with the comparison of Formula 2, and the ignition delay is more sensitive to EGR than the pressure for the larger exponent of (1 − EGR%) than P. For CTRF1, Φ = 1.0, P = 20 bar: τ = 2.28 × 10−10 × (1 − EGR%)−1.511 exp(32.17/(RT )) (4) G


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Energy & Fuels

Figure 9. Reaction pathways for A1CH3/CHX/iC8H18/nC7H16 in CTRF1 at 1000 K, 20 bar, and 20% O2 consumption.

Figure 10. Normalized first-order sensitivity coefficients of the OH radical with respect to pre-exponentials for stoichiometric CTRF1/air mixtures with four EGR loadings at 20 bar, 1000 K.

radical (HO2) have the dominant role without EGR, two active free radicals hydroxide radical (OH) and methoxy (CH3O) are produced through R2655, and the generation of methane through R2656 is a typical chain terminated reaction. Besides,

sensitivity value indicates the promotion effects and a decrease of the ignition delay, while the positive one represents the inhibition of ignition. It can be concluded from Figure 10a that the reactions between methyl radical (CH3) and hydroperoxide H


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Energy & Fuels

Figure 11. Mole fractions of free radicals (H, O, OH, HO2, and CH3) for stoichiometric CTRF1/air mixtures with four EGR loadings at 20 bar, 1000 K.

Figure 12. Rate of production of CO2 for stoichiometric CTRF1/air mixtures with four EGR loadings at 20 bar, 1000 K.

I


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Energy & Fuels

Figure 13. Rate of production of CO for stoichiometric CTRF1/air mixtures with four EGR loadings at 20 bar, 1000 K.

mole fraction of O, H, HO2, and CH3 are doubled. Taking the result of EGR = 0 as an example, as shown in Figure 11a, the concentrations of HO2 and CH3 reach the maximum before the ignition, which enhances the mole fraction of OH rapidly through the reaction R2655: CH3 + HO2 = CH3O + OH to prepare for the ignition. By comparing Figure 11 panels a, b, c, and d, it is remarkable that the relative concentrations (the proportion in all five radicals) of CH3 and HO2 climb with the increase of EGR rates, while the relative concentrations of O and H decline, and the variation of HO2 is the most prominent, in which the concentration rises to be comparable to OH at 60% EGR. This phenomenon may be due to two reasons: most of the HO2 and CH3 radicals are generated through the reactions influenced by the reactions of CO before ignition, such as HCO + O2 = CO + HO2 and CH3CO + H = CH3 + HCO; and the increasing addition of CO2 intensifies the reactions of H + O2(+M) = HO2(+M) and CH3CO(+M) = CH3 + CO(+M) for the large third-body collision coefficient of CO2. Despite the change of the relative concentration of each radical being diverse, the concentrations of all free radicals decrease by orders of magnitude with the increasing EGR rates in which the thermal and dilution effects of EGR play a more dominant role than the chemical effects. To verify the influence of EGR on the reaction flux of CO2 and CO, the rate of production (ROP) analysis for CO2 and CO under conditions of 1000 K, 20 bar, and 0−60% EGR rates are shown in Figure 12 and Figure 13, and only the top four reactions are listed. Figure 12 reveals that the reactions R2730, CO + OH = CO2 + H; R2566, HCCO + O2 ⇒ CO2 + CO + H; and R2615, HOCHO + H ⇒ H2 + CO2 + H contribute the most without EGR, and the total ROP almost coincides with the ROP of R2730. Besides, the reaction R2675, CH2* + CO2 = CH2O + CO occurs and affects the consumption of CO2 with the increasing EGR, and the ROP of R2615 is weakened due to

the third-body reaction R2734 contributes two OH radicals with a second highest negative value, while the consumption of OH, O2, and CH3 makes the reactions of toluene weaken the activity of the mixture and put off the ignition. Compared with Figure 10b, the reactions involved in cyclohexane (CHX) and its subsequent radicals highlight the large sensitivity in both directions. For the six-membered ring, there exists a reaction flux that R + O2 = QOOH ↔ QOOH + O2 ↔ O2QOOH, and the O2QOOH may isomerize and yield carbonylhydroperoxide and a hydroxide radical (OH), then the carbonylhydroperoxide may decompose and produce a second OH,53 so R18. cC6H10O2H-2 + O2 = SOOcC6O2H is a significant chain branching reaction, while the competition reactions R8, R15, and R17 show positive sensitivity value. It is obvious to see that the reactions involved in CH3 and HO2 have correspondingly high sensitivity in all four cases, and there is no clear law of change between the reactions with high sensitivity coefficients and the increasing EGR rates. In addition, the sensitivity values of the third-body reactions are not changed regularly with the rise of CO2 dilution. Notably, the reactions in Figure 10 panels a and c are almost equal, while Figure 10 panels b and d depict similar results, which may be due to the competition of the inhibition effects caused by the growth of the heat capacity and the dilution of the unburned mixture with the promoting effects on reactions that CO2 induced chemically. CO2 + H ↔ OH + CO is the most important reaction in the CO oxidation procedure, and CO2 competes with hydrogen radical (H) with origin fuels to generate OH, improving the activity of the whole system. The concentration of CO2 in the unburned mixture grows with the increase of the EGR rate, strengthening the production of OH. Figure 11 illustrates the concentration history of free radicals in the ignition process of CTRF1 at 20 bar, 1000 K, and EGR rates of 0−60%, and the J


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Energy & Fuels the competition of CO2. It can be concluded from Figure 12d that the disparity between the total ROP and R2730 is enlarged, which proves the chemical effects of CO2 that accelerate other reactions in which CO2 participates. However, the ROP of all reactions for CO2 decreases with the increase of EGR rates and the moment that the maximum appears delayed by comparing all curves, which certifies the predominant role of thermal and dilution effects and the insignificant chemical effects. Similarly, CO is oxidized through the reaction R2730, and yielded mainly through R2570. HCCO + OH ⇔ H2 + CO + CO and R2713. HCO + M ⇔ H + CO + M, which are presented in Figure 13. And the competition of thermal effects and chemical effects leads to the change of CO ROP over the range of conditions with EGR loadings of 0, 20%, 40%, and 60%. Furthermore, the fuel consumption and total rate of production of isooctane, n-heptane, toluene, and cyclohexane are shown in Figures 14 and 15 in terms of the simulated results calculated by the proposed model with the variety of EGR rates from 0 to 60% at condition of Φ = 1.0, T = 1245 K. It can be concluded from Figure 14 that CTRF1 is consumed in the order of cyclohexane, isooctane, n-heptane, and toluene, and remains stable with the variation of EGR loadings, owing to the co-operative effects of their chemical structures and the concentration of each component in CTRF1. Figure 14 also shows the effects that the EGR rate has on the time to consume each component totally. Obviously, all four hydrocarbons need longer time to convert completely, especially for toluene, the time increases from 94, 134, 204, to 367 μs when the EGR rate increases from 0, 20%, 40%, to 60%. In addition, the total rate of production of each component is contradistinguished during the entire fuel consumption period in Figure 15, and the curves of iso-octane, n-heptane, and CHX show similar direction with all EGR loadings, due to the comparable reaction pathways of CHX with alkanes after ring-opening.8 Derived from the analysis above, the CO2 in the exhaust may not influence the chemistry of CTRF1 dissociation intensely, but dilute and modify the heat capacity of the unburned mixture subsequently.

4. CONCLUSION High-pressure shock tube experiments are conducted to investigate the ignition delay of cyclohexane, two quaternary gasoline surrogate fuels CTRF1 and CTRF2 comprising isooctane/n-heptane/toluene/cyclohexane with air (N2/O2 = 3.76:1) at the conditions of 1027−1400 K, 10−20 bar (19 bar for neat cyclohexane), and Φ = 0.5−2.0 (1.5 for neat cyclohexane). Ignition properties are studied when neat CO2 is selected to simulate different EGR loadings (0, 20%, 40%, and 60%) in the CTRF1/air mixture. Additionally, a kinetic model containing 526 species and 2763 reactions is constructed to predict the ignition behavior of the surrogates and to analyze the effects that simulated EGR has on the ignition of CTRF1. Because the experimental results were measured in an ST, the ignition delay shows a negative correlation with the temperatures, pressures, and equivalence ratios for both cyclohexane and the quaternary gasoline surrogates CTRF1 and CTRF2, and the overall activation energy declines with the increase of equivalence ratios under the test conditions described above. Especially, the results in a previous study illustrated that cyclohexane diluted with neat Ar in a high concentration ignited later with the rise of equivalence ratio, which was opposite to the results in these cases.

Figure 14. Fuel consumption of each component with different EGR loadings at 1245 K, 20 bar.

The chemical kinetic model developed for predicting the ignition properties of gasoline surrogate in this work is validated by numerous ignition delay experimental data points of each component and their blends. Even though good agreement is observed under most conditions, there still exist small disparities prompting continuous study of the dominant reactions. Furthermore, because of the lack of experimental results on the surrogates composed of iso-octane, n-heptane, toluene, and cyclohexane, more test facilities besides ST should be used to conduct experiments to verify the prediction ability. The ignition of CTRF1 diluted with CO2 is investigated using an ST to simulate the influence of EGR loadings, and an exponential increase of the ignition delay with the rise of the EGR rate is shown by both experimental and modeled results. On the basis of the chemical kinetic model generated in this study, the reaction flux, sensitivity, and ROP analysis are K


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Energy & Fuels

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Graphic representations of the experimental and modeling results (PDF) Reaction mechanism (TXT) Thermodynamic data (TXT)

AUTHOR INFORMATION

Corresponding Authors

*Tel.: +86-21-34206039. E-mail: [email protected]. *E-mail: [email protected]. ORCID

Xingcai Lu: 0000-0003-3548-6058 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Science Fund for Distinguished Young Scholars (51425602) and the National 973 Major Basic Project (2013CB228405).

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Figure 15. Total rate of production of each component with different EGR loadings at 1245 K, 20 bar.

performed with different EGR loadings. The results show that the chemical effects of CO2 are not evident, for the major consumption paths of each component are not changed and the sensitivity coefficients of reactions affected by the third-body collision present no obvious regular variety with the increasing EGR rates. In addition, the rise of the relative concentration of HO2 and CH3 indicates the negligible chemical effects of CO2, while the predominant role of the thermal and dilution effects is highlighted, due to the decrease of the mole factions of all free radicals by orders of magnitude.

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REFERENCES

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