Kinetic Modeling of the Influence of Cyclohexane on the

Article pubs.acs.org/EF

Cite This: Energy Fuels 2018, 32, 3975−3984

Kinetic Modeling of the Influence of Cyclohexane on the Homogeneous Ignition of a Gasoline Surrogate Fuel J. C. G. Andrae* J A Reaction Engineering, SE-183 32 Täby, Sweden S Supporting Information *

ABSTRACT: The importance of adding a naphthene in the form of cyclohexane in surrogate mixtures to emulate homogeneous ignition of fully blended research gasoline has been examined with kinetic modeling. On the basis of a nonlinear by volume octane blending model and a correlation of anti-knock index to simulated ignition delay time, a quinary surrogate mixture, including cyclohexane, primary reference fuel (PRF), toluene, and diisobutylene (DIB-1), has been formulated that matches the research octane number, motor octane number, and H/C ratio of the target fuel. Simulated ignition delay times for the quinary mixture and a quaternary mixture without cyclohexane have been compared to measured data for the target fuel in a shock tube and rapid compression machine. Kinetic analysis shows that there is an increased production of HO2 during the induction period for the quinary mixture. This leads to an increased OH production/consumption ratio for PRF, toluene, and DIB-1 in the quinary mixture. Simulated homogeneous charge compression ignition experiments at naturally aspirated conditions show that predictions are sensitive to operating conditions. Predictions of intake temperatures needed for phase combustion at top dead center using the quinary mixture are closer to measured data than those using the quaternary mixture when the engine speed is increased from 600 to 1200 rpm and ϕ is >0.2. This is explained by the fact that the cool flame present for the quaternary mixture at 600 rpm disappears when the engine speed is increased to 1200 rpm because the time needed for combustion is not long enough to sustain the low-temperature reactions. formulation.8,10,11,17−19 Ra and Reitz proposed a fourcomponent surrogate mixture to emulate the combustion of a fully blended research gasoline (RD387) involving isooctane, n-heptane, toluene, and cyclohexane (CHX), where the CHX content was as high as 26.4% by mole (22.5% by volume).19 While their kinetic model for the four-component surrogate slightly underpredicted the ignition delay times compared to those simulated with the ternary Stanford A and B surrogates,21 the trend of equivalence ratio variation was better captured by the four-component surrogate, especially at higher temperatures. They did not examine the effect of the engine speed (i.e., residence time) and excluded CHX in the surrogate mixture when simulating homogeneous charge compression ignition (HCCI) experiments. Perez and Boehman found it favorably to include methylcyclohexane (MCH) with an anti-knock index (AKI) of around 74 as a way to increase the reactivity in an optimized gasoline surrogate fuel mixture consisting of the fuels MCH, isooctane, and toluene.11 It has been shown by detailed kinetic modeling that there is a kinetic coupling between aromatics and primary reference fuels that affects the radical pool population and, thereby, controls ignition.12,20 A similar phenomena has recently been demonstrated for naphthenes and aromatics.8 The results described above indicate the importance to further investigate the effect of fuel composition on the autoignition of gasoline surrogate fuels in general and the structure class of

1. INTRODUCTION The shale oil revolution in the United States has changed the world’s energy structure, and at present, many countries are increasing their commercial investments in this field, including China.1,2 When shale oil is upgraded to a clean liquid fuel by hydroconversion, a significant amount of cycloalkanes (naphthenes) are formed.3 As a result of their significant concentrations in conventional liquid fuels and liquid fuels derived from shale oil, the chemical structure class cycloalkanes should be considered in surrogate fuel mixtures when the aim is to emulate the combustion properties of fully blended fuels. The surrogate fuel approach is widely accepted to emulate the complex multicomponent mixtures of hydrocarbon species as fuels used in internal-combustion engines.4 It involves choosing a few representative hydrocarbon species whose overall behavior mimics the characteristics of the target fuel. Once the different chemical structure classes are identified, the next issue involves selecting representative hydrocarbon species for each structure class. To match the empirical measure of autoignition quality for a target gasoline in the form of research octane number (RON) and motor octane number (MON), ternary mixtures, including isooctane, n-heptane, and toluene, may be used.5 To be able to better match fundamental ignition delay data measured in shock tubes (STs) and rapid compression machines (RCMs) and also match the important H/C ratio of the target fuel, multicomponent surrogate fuel mixtures are needed, including olefins and possible cycloalkanes. To represent the structure class olefins, 2-pentene,6,7 1-hexene,7−11 and 2,4,4-trimethyl-1-pentene (DIB-1),12−16 have been used. A few studies have also chosen to include the structure class of cycloalkanes in their gasoline surrogate fuel © 2018 American Chemical Society

Received: December 19, 2017 Revised: February 4, 2018 Published: February 6, 2018 3975

DOI: 10.1021/acs.energyfuels.7b04023 Energy Fuels 2018, 32, 3975−3984

Article

Energy & Fuels Table 1. CHX Subset reaction number 584 585 586 587 588 589 590 591 592 593 594 595 596 597 598 599 600 601 602 603 604 605 606 607 608 609 610 611 612 613 614 615 616 617 618 619 620 621 622 623 624 625 626 627 628 629 630 631 632 633 634 a

reaction

Aa

CHX + H = CYC6H11 + H2 CHX + OH = CYC6H11 + H2O CHX + HO2 = CYC6H11 + H2O2 CHX + O2 = CYC6H11 + HO2 CHX + O = CYC6H11 + OH CHX + CH3 = CYC6H11 + CH4 CHX + C2H3 = CYC6H11 + C2H4 CHX + C2H5 = CYC6H11 + C2H6 CHX = CYC6H11 + H CHX = C2H4 + C2H4 + C2H4 CHX = CH2CHCH3 + CH2CHCH3 CYC6H11 + O2 = CYC6H11O2 CYC6H11 + HO2 = CYC6H11O + OH CYC6H11O2 + O2 = CYOC6H9O + OH + OH CYOC6H9O = OC5H8CHO CYC6H11O2 + O2 = CYC6H9OOH + HO2 CYC6H9O + OH = CYC6H9OOH CYC6H9O = C2H2 + C4H7O CYC6H11O2 + O2 = HOOC5H8CHO + OH OC5H8CHO + OH = HOOC5H8CHO OC5H8CHO = CH2CHCHO + C2H4 + HCO CYCHEXE + H = CYC6H11 CYC6H11O2 (+M) = CYCHEXE + HO2 (+M) low-pressure limit, TROE centering: α = 0.5, T*** = 1 × 10−30, and T* = 1 × 1030 CYCHEXE = CH2CHCHCH2 + C2H4 CYCHEXE + H = CYC6H9 + H2 CYCHEXE + CH3 = CYC6H9 + CH4 CYCHEXE + O = CYC6H9 + OH CYCHEXE + OH = CYC6H9 + H2O CYCHEXE + HO2 = CYC6H9 + H2O2 CYC6H9 + O2 = CYC6H8 + HO2 CYC6H9 + HO2 = CYC6H9O + OH CYC6H8 + H = CYC6H9 CYC6H8 = C6H6 + H2 2CYC6H11O2 = 2CYC6H11O + O2 CYC6H11O = C2H4 + C4H7O CYC6H11 = C6H11-16 C6H11-16 = C6H11-13 C6H11-16 = C2H4 + C4H71−4 C6H11-13 = CH2CHCHCH2 + C2H5 CH2CHCHCH2 + OH = CH2CHC·H2 + CH2O CH2CHCHCH2 + O = CH2CHC·H2 + H + CO C4H71−4 = C2H4 + C2H3 C4H71−4 + O2 = H2CCCH2 + CH2O + OH C4H71−4 + HO2 = C4H7O + OH C4H7O = CH3HCO + C2H3 C4H7O = CH2CHC·H2 + CH2O CYC6H11O2 = C4H6OOH + C2H4 C4H6OOH = C2H4 + C2H2 + HO2 CYC6H11 + C7H16 = CHX + C7H15-2 CYC6H11 + C8H18 = CHX + AC8H17 C6H5CH2 + CHX = C6H5CH3 + CYC6H11

3.44 × 10 2.16 × 105 4.00 × 105 6.72 × 1013 5.72 × 105 3.25 × 105 4.80 × 1012 6.00 × 1011 1.00 × 1011 2.00 × 1013 7.94 × 1016 3.00 × 1012 7.00 × 1012 8.00 × 1012 1.00 × 108 5.20 × 1010 5.00 × 1012 8.24 × 1022 1.00 × 1011 1.81 × 1013 8.00 × 1016 6.25 × 1011 3.50 × 1011 1.00 × 1017 4.00 × 1012 1.00 × 105 2.00 × 10−1 1.59 × 1011 6.44 × 106 1.36 × 104 2.10 × 109 3.00 × 1013 1.25 × 1012 4.00 × 1012 1.40 × 1016 2.68 × 1017 1.34 × 1012 9.30 × 1014 6.31 × 1012 5.35 × 1012 2.80 × 1012 6.00 × 108 1.26 × 1013 7.29 × 1029 7.00 × 1012 7.94 × 1014 1.01 × 1018 4.79 × 1016 1.00 × 1011 5.01 × 1010 5.01 × 1010 5.01 × 1010 7

na

Ea

reference

2.35 2.5 2.5 0.0 2.7 2.3 0.0 0.0 1.08 0.0 0.0 0.0 0.0 0.0 0.9 0.0 0.0 −2.7 0.1 0.0 −1.6 0.51 1.2 1.2 0.0 2.5 3.5 0.7 2.0 2.5 0.0 0.0 0.5 0.0 −1.6 −1.6 0.55 −0.45 0.43 0.56 0.0 1.4 0.4 −5.7 0.0 0.0 −1.4 −1.2 0.0 0.0 0.0 0.0

4124.0 −1133.0 14147.4 48210 2106 7287 16800 10400 86759 57400 86281 0.0 −1000 14495.3 5900.0 7000 0.0 32770 10400 0.0 31560.0 2620.0 35000 37000 57400.0 −1912.0 4046.1 3107.1 −1434 10113.8 0.0 −1000 1500 57400 1860 30410 32982.8 22700 31788 39436 −900 900 38958 21450 −1000 19000 30840 34640 37000 11200 11200 11200

this work 19 this work 19 19 19 19 19 27 19 28 19 19 19 19b this work this work 19b this workb 19 19 19 this work 19 19 19 19 this work 19 19 this work 19 19 19 19b 27b this work 27 27 21 21 27 19 19 19 19 19b 19 19 19 19

Forward rate constant kf = ATn exp(−E/RT), with A in units of mol cm s and E in units of cal/mol. bCHX ring-opening reaction.

naphthenes in particular. Specifically, the impact of partly replacing the branched- and straight-chain alkane contents by a cycloalkane on the autoignition chemistry has not been isolated in kinetic modeling studies. Therefore, in this work, the impact of adding the naphthene CHX to a quaternary mixture consisting of primary reference fuel (PRF), toluene, and DIB-1 to emulate the

homogeneous ignition of fully blended research gasoline RD387 is studied by kinetic modeling. Both the quaternary and quinary surrogate fuel mixtures have similar RON, MON, and H/C ratio as the target fuel. Ignition delay times measured in the ST and RCM for the target fuel are compared to simulated ones, and kinetic analysis is conducted to interpret the results. Finally, 3976


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Energy & Fuels HCCI experiments conducted with the same target fuel at naturally aspirated conditions are compared to simulations using the two surrogate fuels. The effect of the engine speed, fuel/air ratio, and intake temperature on the autoignition is studied and explained in terms of fuel chemistry.

2. SIMULATIONS The chemical kinetics software Cantera is used for simulations in this work.22 For ST ignition delay experiments, a constant volume, adiabatic condition was assumed using the compressed temperature and pressure as initial conditions. For undiluted conditions, the ignition delay time is determined as the time required to rise the temperature to 400 K over the initial temperature. For ST experiments at diluted conditions, the time of significant increase in the OH concentration was used to define

Figure 4. Ignition time measurements for CHX/synthetic air mixtures at 1.25 MPa and ϕ = 1.031,32 and comparison to ( and - - -) prediction of the kinetic mechanism in this work. Conditions for heat loss simulations: clearance volume, 37.9 cm3; bore, 0.05 m; and α, 2 × 10−6 m2/s.

Figure 1. Measured CHX ignition delay times behind reflected shock waves by Hong et al.29 compared to predictions (lines) by the model in this work using a constant volume assumption, with mixture compositions of 0.444 and 0.222% fuel, 4% O2, and balance gas Ar: ( and ■) ϕ = 1.0 and p = 3 atm, (- - - and ●) ϕ = 1.0 and p = 1.5 atm, and (-·- and □) ϕ = 0.5 and p = 1.5 atm.

Figure 5. Comparison of (, - - -, and ···) simulated and measured species profiles33 in a jet-stirred reactor, with 0.1% CHX/O2/N2, τ = 0.5 s, ϕ = 1.0, and p = 1.013 MPa, for (+) O2, (●) CHX, (◆) H2, (■) CO, (□) CO2, and (▲) CH2O.

Figure 2. (▲) H2O and (●) OH time histories obtained during the oxidation of CHX at 1441 K, 2.2 atm, and mixture composition of 467 ppm of fuel, 4200 ppm of O2, and balance gas Ar.29 ( and - - -) Simulations were performed using a constant volume assumption.

Figure 6. Ignition time measurements for CHX/synthetic air mixtures behind reflected shock waves by Daley et al.32 with comparison to (, -·-, and - - -) predictions of the kinetic mechanism in this work using a constant volume assumption: ( and ■) ϕ = 1.0, (-·- and ▲) ϕ = 0.5, and (- - - and ●) ϕ = 0.25.

Figure 3. Comparison of (■) measured30 and () modeled laminar burning velocities. 3977


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the ignition delay. At undiluted conditions, the OH definition and temperature increase definition give similar ignition delay times. For RCM experiments, in addition to a constant volume assumption, heat loss after compression was modeled as an increase of the volume with time according to the relationship ⎛B ⎞2 V (t ) = π ⎜ + δ ⎟ (h + 2δ) ⎝2 ⎠

In eq 1, B is the bore, δ is the thermal boundary layer, and h is the clearance height. The boundary layer is defined as (αt)0.5, where α is the thermal diffusivity.23 HCCI experiments have been simulated using a single-zone modeling approach, which is a reasonable model assumption when the main parameter of interest is combustion timing. This approach treats the incylinder charge as a single lumped mass with uniform composition and thermodynamic properties, which is compressed and expanded adiabatically using the standard slider−crank relationship.24 Sensitivity and rate-of-production analyses for constant volume, adiabatic, and spatially homogeneous batch reactor simulations are conducted with the program package KINALC.25

Figure 7. Measured AKI for 18 fuels as a function of the simulated ignition delay, with p = 25 atm, T = 825 K, and ϕ = 1.0.

Table 2. Gasoline Surrogate Fuels Used in This Work fuel, formula isooctane, C8H18 n-heptane, C7H16 toluene, C6H5CH3 DIB-1, JC8H16 CHX, Cy-C6H12 τ: p = 25 atm, T = 825 K, and ϕ = 1.0b AKIc S (RON − MON)d RON MON H/C

quaternarya (vol %/mol %) 51.0/43.5 18.0/17.3 26.4/35.0 4.6/4.1

quinary (vol %/mol %)

4.250 × 10−3 s

41.7/33.6 12.7/11.5 26.4/33.0 4.6/3.9 14.6/18.0 4.355 × 10−3 s

87.0 6.9 90.4 83.5 1.882

87.4 7.2 91.0 83.8 1.850

(1)

3. KINETIC MODEL As a representative species for cycloalkanes, CHX has been added to the starting mechanism in the form of a semi-detailed kinetic model for quaternary mixtures involving PRF, toluene, and DIB-1.16 Table 1 shows the CHX subset that was adopted from Ra and Reitz.19 It should be noted that the base mechanism in this work is very different from that used in that work, and they only considered measured ignition delay data in the model validation. With this fact together with that in this work, a much more comprehensive validation has been conducted. Some rate constants have been updated to improve model predictions (see Table 1). Moreover, by revising the rate constants for the reactions CH2CHCHCH2 + OH = CH2CHC·CH2 + H2O and CH2CHCHCH2 + O2 = CH2CHC· CH2 + HO2 in the starting mechanism to the those used by Ra and Reitz

a

The composition is the same as in ref 16. bConditions based on the work by Mehl et al.7 cCalculated using the correlation in Figure 7. d Using the Ghosh correlation and parameters for isooctane, n-heptane, toluene, and DIB-1 as in ref 16.

Figure 8. Comparison of measured ignition delays of fully blended gasoline RD387 and gasoline surrogate fuels to ( and - - -) simulated total ignition delay using surrogate fuel compositions as in Table 2: (○) ST gasoline,21 (●) RCM gasoline,6 (△) ST PRF90,39 (×) ST TPRF91,40 (+) ST PRF91,41 and (◇) ST isooctane,42 with ( and - - -) constant volume assumption (α = 0) and ( and - - - with □) volume expansion with clearance volume of 12.95 cm3, bore of 5.08 cm, and α of 2 × 10−6 m2/s. 3978


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to 20 bar using the pressure exponent 0.75 by Shen et al.43 Model predictions are in line with the introduced ST data, suggesting that the facility effect of the RCM makes measured ignition times appear longer in the NTC region compared to if they had been measured under more well-defined conditions (ST). These facility effects may be caused by temperature heterogeneities.44 5.2. Simulation of HCCI Experiments. To investigate the effect of time available for combustion when the temperature and fuel/air ratio are varied, HCCI engine experiments with the target fuel have been simulated using the two surrogate fuels. Simulations were performed using a single-zone model, where combustion phasing was defined as the moment when the concentration of CO peaked. To model heat loss, simulations were performed with a lower compression ratio than the geometrical of 18.16,46 Compression ratios were increased with an increasing engine speed to reflect decreased time available for heat transfer, hotter combustion chamber surfaces, the “ram” effect, and charge heating as a result of increased turbulent dissipation of kinetic energy.24,45 Figure 9a shows that there is no significant difference between predictions using the quaternary and quinary mixture at 600 rpm. However, when the engine speed is increased to 1200 and

to improve results at jet-stirred reactor conditions for neat CHX and updating the thermochemical data for hexyl and butyl radicals that are relevant to the CHX chemistry (C6H11-13, C6H11-16, and C4H71−4) to the those by Zhang et al.26 led to improvement of predictions at hightemperature and low-pressure conditions. The overall model, which consists of 140 species and 634 reactions, may be obtained in electronic format upon request. Figures 1−3 show the validation of the CHX subset against hightemperature data at low pressure.29,30 The predicted ignition delay in Figure 1 is defined as the time when the slope of the OH radical mole fraction is at a maximum. Laminar burning velocities were simulated using multicomponent transport properties with refine criteria ratio = 3, slope = 0.05, curve = 0.05, and prune = 0.01. This resulted in 300−400 grid points for a converged solution. Overall, the predictions are in satisfactory agreement with measurements. Especially, the sensitivity of model predictions to changes in the fuel/air ratio and pressure is improvement compared to some of the published models for neat CHX (Figure 1 compared to Figure 3 in ref 29). Figures 4−6 show predictions at higher pressures (>1.0 MPa) compared to measured data in the ST, RCM, and jet-stirred reactor.31−33 Generally, the kinetic model reproduces the measured data for neat CHX qualitatively and in most cases quantitatively. Results of the simulations demonstrate that the kinetic model overall correctly reflects CHX oxidation and heat release, allowing for the use of the developed mechanism for investigation of surrogate mixtures involving CHX as one of the components.

4. SURROGATE FUEL FORMULATION In this work, the target fuel is research gasoline RD387 with a reported AKI of 87, RON of 91, MON of 83, and H/C ratio of 1.87.6,21 Moreover, the chemical composition is comprised of 68−73 vol % saturates, 23−26 vol % aromatics, and 4−5 vol % olefins.6 The starting surrogate for RD387 was a quaternary mixture with PRF, toluene, and DIB-1.16 To formulate a quinary mixture including CHX, a nonlinear by volume octane blending model by Ghosh et al.34 was used together with a method by Mehl et al.7 to correlate the AKI of a fuel to simulated ignition delay time. Figure 7 shows AKI plotted against ignition delay time for 18 test fuels in the literature with known MON and RON.10,12,34−38 More details are provided in the Supporting Information. Table 2 shows the resulting gasoline surrogate fuels. The significant compositional difference between the fuels are that, in the quinary mixture, the PRF content has been partly replaced by CHX.

5. RESULTS AND DISCUSSION 5.1. Simulation of Ignition Data from ST and RCM Experiments. Figure 8 shows simulated ignition delay times for the quinary mixture together with those for the quaternary mixture. Predictions using both surrogates show good agreement when compared to measured delay times and are close to each other. This is not surprising because, in addition to a comprehensively validated kinetic model, both mixtures have been formulated to mimic MON, RON, and H/C as well as the chemical composition of the target fuel. Nevertheless, the quinary mixture, including CHX, is more reactive than the quaternary mixture for temperatures lower than around 830 K and shows better agreement against measured delay times in RCM. This is reinforced for ϕ = 0.5 and 0.3 (see panels c−f of Figure 8). Figure 8 shows that the effect of heat loss is most significant at 20 bar and for lean fuel/air ratios. At especially 20 bar, the measured RCM data show a significant negative temperature coefficient (NTC) behavior not reproduced in the simulations. This was examined further by plotting ignition data measured in well-defined ST experiments for gasoline surrogate fuels with similar resistance to autoignition as RD387.39−42 The isooctane ignition data in Figure 8c by Hartmann et al.42 have been scaled

Figure 9. Required intake temperatures at an intake pressure of 0.1 MPa to phase combustion at TDC in a HCCI engine.45 Simulations were conducted using surrogate fuel compositions described in Table 2. 3979


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Energy & Fuels Table 3. Sensitivity of the Calculated Temperature on Rate Coefficientsa number 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 a

Sj̃ =

kj ∂T T ∂kj

9.12 × 10−3 6.64 × 10−3 5.89 × 10−3 4.54 × 10−3 3.68 × 10−3 3.32 × 10−3 3.25 × 10−3 −3.20 × 10−3 2.88 × 10−3 2.69 × 10−3 −2.61 × 10−3 2.44 × 10−3 −2.35 × 10−3 2.13 × 10−3 −1.66 × 10−3 −1.64 × 10−3 1.62 × 10−3 1.51 × 10−3 1.34 × 10−3 1.16 × 10−3

reaction number

reaction

452 470 437 426 455 3 471 14 454 475 487 425 456 451 32 486 453 460 27 424

AC8H16OOH-B + O2 ⇄ AC8H16OOH-BO2 C6H5CH2OOH + C7H15-2 ⇄ C6H5CH2OO + C7H16 OC7H13O2H → OC7H13O + OH C7H16 + OH → C7H15-2 + H2O OC8H15OOH → OC8H15O + OH C6H5CH3 + O2 ⇄ C6H5CH2 + HO2 C6H5CH2OOH ⇄ C6H5CH2O + OH C6H5CH3 + OH ⇄ C6H4CH3 + H2O C8H18 + OH → AC8H17 + H2O C6H5CH2OOH + AC8H17 ⇄ C6H5CH2OO + C8H18 JC8H16 + OH ⇄ JC8H15-B + H2O C7H16 + OH → C7H15-1 + H2O AC8H17 + O2 ⇄ JC8H16 + HO2 AC8H17OO ⇄ AC8H16OOH-B 2C6H5CH2 ⇄ BIBENZYL JC8H16 + OH ⇄ JC8H15-A + H2O AC8H16OOH-BO2 → OC8H15OOH + OH C8H18 + HO2 ⇄ AC8H17 + H2O2 C6H5CH2 + HO2 ⇄ C6H5CH2O + OH C7H16 + HO2 → C7H15-2 + H2O2

Quaternary mixture, T0 = 735 K, p0 = 20 bar, ϕ = 1.0, and 1% isooctane conversion.

Table 4. Sensitivity of the Calculated Temperature on Rate Coefficientsa number 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 a

Sj̃ =

kj ∂T T ∂kj

1.82 × 10−2 −1.19 × 10−2 −1.18 × 10−2 8.62 × 10−3 7.26 × 10−3 5.68 × 10−3 4.26 × 10−3 3.72 × 10−3 3.67 × 10−3 3.49 × 10−3 3.12 × 10−3 3.11 × 10−3 3.06 × 10−3 2.68 × 10−3 −2.66 × 10−3 2.34 × 10−3 2.30 × 10−3 −2.24 × 10−3 −2.21 × 10−3 2.01 × 10−3

reaction number

reaction

597 599 606 452 602 470 437 586 3 455 471 14 426 475 4 27 460 487 456 451

CYC6H11O2 + O2 ⇄ CYOC6H9O + 2OH CYC6H11O2 + O2 ⇄ CYC6H9OOH + HO2 CYC6H11O2 (+M) ⇄ CYCHEXE + HO2 (+M) AC8H16OOH-B + O2 ⇄ AC8H16OOH-BO2 CYC6H11O2 + O2 ⇄ HOOC5H8CHO + OH C6H5CH2OOH + C7H15-2 ⇄ C6H5CH2OO + C7H16 OC7H13O2H → OC7H13O + OH CHX + HO2 ⇄ CYC6H11 + H2O2 C6H5CH3 + O2 ⇄ C6H5CH2 + HO2 OC8H15OOH → OC8H15O + OH C6H5CH2OOH ⇄ C6H5CH2O + OH C6H5CH3 + OH ⇄ C6H4CH3 + H2O C7H16 + OH → C7H15-2 + H2O C6H5CH2OOH + AC8H17 ⇄ C6H5CH2OO + C8H18 C6H5CH3 + OH ⇄ C6H5CH2 + H2O C6H5CH2 + HO2 ⇄ C6H5CH2O + OH C8H18 + HO2 ⇄ AC8H17 + H2O2 JC8H16 + OH ⇄ JC8H15-B + H2O AC8H17 + O2 ⇄ JC8H16 + HO2 AC8H17OO ⇄ AC8H16OOH-B

Quinary mixture, T0 = 735 K, p0 = 20 bar, ϕ = 1.0, and 1% isooctane conversion.

involving n-heptane and isooctane show the highest positive sensitivity, while hydrogen abstraction with OH for DIB-1 and toluene shows negative sensitivity. n-Heptane and to a lesser extent isooctane promote the ignition of less reactive toluene through reactions R470 and R475. According to the rate-ofproduction analysis for peroxy benzyl at the same conditions, over 80% of the consumption proceeds through those two reactions. Benzyl hydroperoxide is decomposed to alkoxy benzyl and OH in reaction R471, contributing to 4% of the OH production (see Table 5). Table 5 shows that, in the quaternary mixture, reactions originating from n-heptane contribute more to the production of

1800 rpm, predictions using the quinary mixture are in closer agreement with measurements at ϕ > 0.2 (see panels b and c of Figure 9). 5.3. Kinetic Analysis. To explain the results shown in Figures 8 and 9, a kinetic analysis has been conducted. Tables 3 and 4 show the result of a sensitivity analysis of the calculated temperature on rate coefficients at constant volume conditions and 1% isooctane conversion, and Tables 5 and 6 show the rateof-production analysis of the OH radical at the same conditions. In Tables 3 and 4, a positive sensitivity for a particular reaction indicates a promoting effect and vice versa. Table 3 shows that, according to the kinetic model, the quaternary mixture reactions 3980


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Energy & Fuels Table 5. Rate of Production/Consumption of OHa number 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 a

contributionb (mol cm−3 s−1) 6.56 × 10−5 −5.05 × 10−5 −4.09 × 10−5 −4.03 × 10−5 3.50 × 10−5 2.88 × 10−5 −2.17 × 10−5 1.86 × 10−5 1.13 × 10−5 −1.11 × 10−5 9.82 × 10−6 −9.71 × 10−6 −7.62 × 10−6 7.61 × 10−6 6.38 × 10−6 4.98 × 10−6 −4.63 × 10−6 −1.46 × 10−6 1.36 × 10−6 −1.28 × 10−6

reaction number

reaction

436 454 4 426 453 455 425 27 437 487 542 14 486 471 512 511 488 37 194 269

HO2C7H13O2H → OC7H13O2H + OH C8H18 + OH → AC8H17 + H2O C6H5CH3 + OH ⇄ C6H5CH2 + H2O C7H16 + OH → C7H15-2 + H2O AC8H16OOH-BO2 → OC8H15OOH + OH OC8H15OOH → OC8H15O + OH C7H16 + OH → C7H15-1 + H2O C6H5CH2 + HO2 ⇄ C6H5CH2O + OH OC7H13O2H → OC7H13O + OH JC8H16 + OH ⇄ JC8H15-B + H2O IC4H7 + HO2 ⇄ IC4H7O + OH C6H5CH3 + OH ⇄ C6H4CH3 + H2O JC8H16 + OH ⇄ JC8H15-A + H2O C6H5CH2OOH ⇄ C6H5CH2O + OH JC8H15-D + HO2 ⇄ JC8H15O−D + OH JC8H15-B + HO2 ⇄ JC8H15O-B + OH JC8H16 + OH ⇄ JC8H15-D + H2O C6H5CHO + OH ⇄ H2O + C6H5CO CH2CHC·H2 + HO2 ⇄ CH2CHCHO + H + OH CH2O + OH ⇄ HCO + H2O

34.30% P 26.40% C 21.40% C 21.10% C 18.30% P 15.10% P 11.40% C 9.70% P 5.90% P 5.80% C 5.10% P 5.10% C 4.00% C 4.00% P 3.30% P 2.60% P 2.40% C 0.80% C 0.70% P 0.70% C

Quaternary mixture, T0 = 735 K, p0 = 20 bar, ϕ = 1.0, and 1% isooctane conversion. bNet OH production rate: 1.665 × 10−8 mol cm−3 s−1.

Table 6. Rate of Production/Consumption of OHa number 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 a

contributionb (mol cm−3 s−1) −7.65 × 10−5 4.98 × 10−5 4.60 × 10−5 −4.12 × 10−5 −4.08 × 10−5 2.86 × 10−5 −2.84 × 10−5 2.63 × 10−5 2.34 × 10−5 −1.53 × 10−5 −1.10 × 10−5 9.81 × 10−6 9.81 × 10−6 −9.71 × 10−6 8.73 × 10−6 −7.59 × 10−6 7.57 × 10−6 7.32 × 10−6 7.23 × 10−6 7.00 × 10−6 6.64 × 10−6 −4.61 × 10−6 −1.71 × 10−6 −1.70 × 10−6

reaction number

reaction

585 597 436 454 4 453 426 27 455 425 487 602 603 14 512 486 437 471 588 511 542 488 37 253

CHX + OH ⇄ CYC6H11 + H2O CYC6H11O2 + O2 ⇄ CYOC6H9O + 2OH HO2C7H13O2H → OC7H13O2H + OH C8H18 + OH → AC8H17 + H2O C6H5CH3 + OH ⇄ C6H5CH2 + H2O AC8H16OOH-BO2 → OC8H15OOH + OH C7H16 + OH → C7H15-2 + H2O C6H5CH2 + HO2 ⇄ C6H5CH2O + OH OC8H15OOH → OC8H15O + OH C7H16 + OH → C7H15-1 + H2O JC8H16 + OH ⇄ JC8H15-B + H2O CYC6H11O2 + O2 ⇄ HOOC5H8CHO + OH OC5H8CHO + OH ⇄ HOOC5H8CHO C6H5CH3 + OH ⇄ C6H4CH3 + H2O JC8H15-D + HO2 ⇄ JC8H15O−D + OH JC8H16 + OH ⇄ JC8H15-A + H2O OC7H13O2H → OC7H13O + OH C6H5CH2OOH ⇄ C6H5CH2O + OH CHX + O ⇄ CYC6H11 + OH JC8H15-B + HO2 ⇄ JC8H15O-B + OH IC4H7 + HO2 ⇄ IC4H7O + OH JC8H16 + OH ⇄ JC8H15-D + H2O C6H5CHO + OH ⇄ H2O + C6H5CO OH + HO2 ⇄ H2O + O2

31.50% C 20.50% P 18.90% P 17.00% C 16.80% C 11.80% P 11.70% C 10.80% P 9.60% P 6.30% C 4.50% C 4.00% P 4.00% P 4.00% C 3.60% P 3.10% C 3.10% P 3.00% P 3.00% P 2.90% P 2.70% P 1.90% C 0.70% C 0.70% C

Quinary mixture, T0 = 735 K, p0 = 20 bar, ϕ = 1.0, and 1% isooctane conversion. bNet OH production rate: 1.700 × 10−8 mol cm−3 s−1.

sensitive reaction and contributes around 20% of the OH production. The fact this reaction involves O2 would explain why the promoting effect of CHX at T < 830 K is reinforced for leaner mixtures (see Figure 8). Table 6 shows that, although CHX itself is neutral in OH production at the specific temperature, it has an impact on the OH production/consumption ratios for some of the other fuels when compared to the quaternary mixture. The ratio for the PRFs is not affected much, while the ratio for toluene is increased by around 28% and the ratio for DIB-1 is increased

OH (40% P) than its consumption (32% P). This is also the case for isooctane (33% P versus 26% C). DIB-1 is slightly net consuming OH (11% P versus 12% C), where DIB-1 is responsible for OH production in reaction R542 as IC4H7 is produced by decomposition of the DIB-1 radical JC8H15-A in reaction R507. Toluene is clearly a net consumer of OH (14% P versus 27% C). Tables 4 and 6 show the implications by introducing CHX as a fuel component in the gasoline surrogate fuel. The chain-branching reaction CYC6H11O2 + O2 ⇄ CYOC6H9O + 2OH is the most 3981


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Figure 10 shows the evolution of the temperature, HO2, and OH at the same initial conditions. Although the quaternary mixture exhibit a larger first-stage OH peak than the quinary mixture as a result of a higher content of PRF, the HO2 production rate for the quinary mixture is increased in the induction period. This is due to reactions R599 and R606, where the contribution from reaction R606 increases with the temperature as opposed to reaction R599 and has a direct promoting effect on toluene reactivity and OH production through reaction R27 and to a lesser extent on DIB-1 reactivity through reactions R511 and R512 (cf. Tables 5 and 6). The importance of reaction R27 for autoignition of fuel mixtures has been discussed in previous publications.12,20 Figure 11 shows the impact of CHX on the OH production/ consumption ratio for the individual fuel components as a function of the temperature and extends the rate-of-production analysis at 735 K, shown in Tables 5 and 6. As an example, for toluene at 735 K in the quaternary mixture, Table 5 gives (9.7 + 4)/ (21.4 + 5.1 + 0.8) = 0.50, and for isooctane in the quaternary mixture, Table 5 gives (18.3 + 15.1)/26.4 = 1.27. Similar calculations can be performed at other temperatures and fuels. While the ratio for CHX decreases with the temperature, there is a significant promoting effect on the ratio for the other fuels, which explains the increased reactivity of the quinary mixture (see Figures 8 and 10). The HCCI simulations show that, when adding CHX to a quaternary mixture and at the same time partly replacing isooctane and n-heptane, predictions using the quinary mixture are closer to measured data at higher fuel/air ratios and engine speeds (see Figure 9) Therefore, the residence time needed for reactions leading to autoignition is sensitive to fuel composition at those conditions. Figures 12 and 13 show evolution of the temperature, CO concentration, and conversion of individual fuel components at two fuel/air ratios and engine speeds. The observed “negative” conversion for DIB-1 (JC8H16) from around −15 to −5 crank angle (CA) after top dead center (ATDC) can be explained by the coupling of the isooctane submechanism with that of DIB-1 through reaction R456: AC8H17 + O2 ⇆ JC8H16 + HO2. This

by around 7%. Table 6 also shows that, in comparison to the quaternary mixture (see Table 5), the contribution from the two most important OH consumption reactions originating from toluene (reactions R4 and R14) is decreased and the contribution from the most important chain-branching reaction (reaction R27) is increased.

Figure 10. Evolution of the temperature, HO2, and OH, with T0 = 735 K, p0 = 2.0 MPa, and ϕ = 1.0, for a constant volume simulation.

Figure 11. OH production/destruction ratio for different fuel components as a function of the initial temperature: (●) n-heptane, (◇) isooctane, (▲) toluene, (■) DIB-1, and (+) CHX, with () quinary mixture and (- - -) quaternary mixture, for a constant volume simulation, with p0 = 2.0 MPa, ϕ = 1.0, and 1% conversion of isooctane.

Figure 12. Simulated fuel conversion as a function of the engine speed: () −600 rpm and (- - -) −1200 rpm, for a quaternary mixture (Table 2). 3982


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Figure 13. Simulated fuel conversion as a function of the engine speed: () −600 rpm and (- - -) −1200 rpm, for a quinary mixture (Table 2).

ratio for the individual fuels, thereby decreasing the ignition delay for the quinary mixture. (3) The kinetic analysis further shows that fuels with larger first stage OH peaks are not always less resistant to autoignition because it depends upon the time available for combustion. (4) HCCI simulations show that the increased reactivity introduced by adding CHX and partly removing PRF makes predictions with the quinary mixture less sensitive to engine speed variation.

results in a net production of DIB-1. However, upon further compression, leading to increased temperature (and pressure), the net consumption of DIB-1 resumes and is completed at the same moment as isooctane. At the leaner fuel/air ratio (ϕ = 0.16), there is no observable low-temperature heat release for either the quaternary or quinary fuel mixture (see Figures 12b and 13b). This can be correlated to the higher temperatures needed (>950 K) to ignite such lean mixtures (see Figures 12a and 13a). It can also be seen that CHX is converted faster than n-heptane in the quinary mixture and toluene is converted faster than isooctane for both mixtures. At the richer fuel/air ratio (ϕ = 0.30), there is a significant difference in the shape of the conversion curves for the quaternary mixture when the two engine speeds are compared. At 600 rpm, there is an observed cool flame behavior, which makes the quaternary mixture more reactive (see Figure 12d). This is also evident from the temperature profile in Figure 10. The cool flame disappears when the engine speed is increased to 1200 rpm because the residence time is not long enough to sustain the lowtemperature reactions. Contrary to the quaternary mixture, the quinary mixture shows no cool flame behavior at 600 rpm and ϕ = 0.3 (see Figure 13d). This can also be seen from the temperature profile in Figure 10. The increased reactivity introduced by adding CHX and partly removing PRF makes the quinary mixture less sensitive to engine speed variation. Therefore, for ϕ > 0.2, the quaternary mixture with a higher content of PRF and absence of CHX requires higher intake temperatures at higher engine speeds to phase combustion at top dead center (TDC) (see Figure 9).

■

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.7b04023. More details for Figure 7 (PDF)

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

J. C. G. Andrae: 0000-0001-5372-5556 Notes

The author declares no competing financial interest.

■

REFERENCES

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6. CONCLUSION (1) Adding CHX to a quaternary mixture including PRF, toluene, and DIB-1 and partly replacing the PRF content increases the overall reactivity and improves model predictions at RCM conditions. (2) The kinetic analysis shows that there is an increased production rate of HO2 during the induction period for the quinary mixture. This leads to an increased OH production/consumption 3983


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