Autoignition of toluene and benzene at elevated pressures in a rapid compression machine

Combustion and Flame 150 (2007) 355–368www.elsevier.com/locate/combustflame

Autoignition of toluene and benzene at elevated pressuresin a rapid compression machine

Gaurav Mittal, Chih-Jen Sung ∗

Department of Mechanical and Aerospace Engineering, Case Western Reserve University, Cleveland, OH 44106, USA

Received 28 July 2006; received in revised form 20 March 2007; accepted 25 April 2007

Available online 21 June 2007

Abstract

Autoignition of toluene and benzene is investigated in a rapid compression machine at conditions relevant toHCCI (homogeneous charge compression ignition) combustion. Experiments are conducted for homogeneousmixtures over a range of equivalence ratios at compressed pressures from 25 to 45 bar and compressed tempera-tures from 920 to 1100 K. Experiments varying oxygen concentration while keeping the mole fraction of tolueneconstant reveal a strong influence of oxygen in promoting ignition. Additional experiments varying fuel molefraction at a fixed equivalence ratio show that ignition delay becomes shorter with increasing fuel concentration.Moreover, autoignition of benzene shows significantly higher activation energy than that of toluene. In addition,the experimental pressure traces for toluene show behavior of heat release significantly different from the resultsof Davidson et al. [D.F. Davidson, B.M. Gauthier, R.K. Hanson, Proc. Combust. Inst. 30 (2005) 1175–1182].Predictability of various detailed kinetic mechanisms is also compared. Results demonstrate that the existingmechanisms for toluene and benzene fail to predict the experimental data with respect to ignition delay and heatrelease. Flux analysis is further conducted to identify the dominant reaction pathways and the reactions responsiblefor the mismatch of experimental and simulated data.© 2007 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

Keywords: Toluene; Benzene; Autoignition; Rapid compression machine

1. Introduction

Commercial gasoline contains aromatics rangingfrom 20 to 50%, among which toluene is predomi-nant. Therefore, toluene is recognized as an essentialcomponent representing aromatics in gasoline surro-gates. Toluene is also formed from the oxidation andpyrolysis of hydrocarbons, leading to the subsequentformation of polycyclic aromatic hydrocarbons andeventually soot.

* Corresponding author. Fax: +1 (216) 368 6445.E-mail address: [email protected] (C.-J. Sung).

0010-2180/$ – see front matter © 2007 The Combustion Institute.doi:10.1016/j.combustflame.2007.04.014

The chemical kinetic models for toluene havebeen developed mostly with emphasis on experimen-tal data at low pressures and high temperatures [1–13]. Bounaceur et al. [14] investigated low-tempe-rature oxidation of toluene at atmospheric pressure.Recently, Sivaramakrishnan et al. [15,16] obtainedtoluene oxidation data in a single-pulse shock tubeat high pressures up to 600 bar and developed a de-tailed kinetic model for toluene oxidation under theseconditions. In general, the chemistry of toluene athigh pressures and low temperatures, which is rele-vant to HCCI combustion, has not been well validatedand hence needs further development. Except therapid compression machine (RCM) measurements by

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356 G. Mittal, C.-J. Sung / Combustion and Flame 150 (2007) 355–368

Roubaud et al. [17] at 907 K and 20 bar, and the shocktube measurements by Davidson et al. [18] at interme-diate temperatures (970–1250 K) and high pressures(13–54 bar), to the authors’ knowledge there is noother investigation of toluene autoignition in the sim-ilar pressure and temperature range. Due to the lackof such data, the existing models remain invalidated.Davidson et al. [18] also noted that the predictionsfor their experimental data using the existing mech-anisms failed to capture the significant difference inthe experimental activation energies and delay timesin the stoichiometric and fuel-lean mixtures.

Recognizing the lack of kinetic information atconditions of high pressures and low-to-intermediatetemperatures, this work focuses on toluene autoigni-tion in this regime. Experiments are conducted for ho-mogeneous toluene/O2/N2/Ar mixtures over a rangeof pressures, equivalence ratios, and dilution levels.In addition, since benzene chemistry is a submecha-nism in toluene kinetics, ignition delay measurementsare also conducted for benzene. Experimental resultsare subsequently compared with calculations usingvarious detailed reaction mechanisms available in theliterature. Kinetic analysis is also carried out to delin-eate the controlling chemistry and highlight the defi-ciencies in the existing mechanisms.

2. Experimental and computational specifications

Experiments are conducted in a rapid compressionmachine. The RCM system consists of a driver piston,a reactor piston, a hydraulic motion control chamber,and a driving air tank. The driver cylinder has a boreof 5 inches and the reactor cylinder bore is 2 inches.The machine is pneumatically driven and hydrauli-cally stopped. The stroke can be varied between 7 and10 inches, by adjusting the spacers on the hydrauliccylinder. The clearance volume of the present RCMis also adjustable and can be varied by using splitshims between the hydraulic cylinder head and thereactor cylinder. The cylindrical reaction chamber isequipped with sensing devices for measuring pressureand temperature, gas inlet/outlet ports for preparingthe reactant mixture, and quartz windows for opticalaccess. Dynamic pressure during compression is mea-sured using a Kistler 6125B transducer with 5010Bcharge amplifier. The machine incorporates an opti-mized creviced piston head design to promote the ho-mogeneous reaction zone. Homogeneous mixture ofreactants is prepared manometrically inside a mixingtank equipped with a magnetic stirrer. When prepar-ing fuel/oxidizer mixtures at room temperature, thefuel partial pressure is kept less than the correspond-ing saturation pressure to prevent condensation. Inaddition, by monitoring the pressure of individually

Table 1List of compositions (partial pressures in Torr) of gas mix-tures prepared

Toluene O2 N2 Ar φ Xtoluene

16.0 144.0 168.0 1336.0 1 0.0096213.5 162.0 94.5 1134.0 0.75 0.0096213.5 243.0 0.0 1147.5 0.5 0.0096219.0 171.0 0.0 1276.8 1 0.012958.2 73.8 204.9 895.8 1 0.006938.2 73.8 267.2 1003.6 1 0.00606

Benzene O2 N2 Ar φ Xbenzene

27.5 206.3 432.9 2193.4 1 0.0096227.5 412.5 193.6 2226.4 0.5 0.00962

Xtoluene and Xbenzene represent the mole fractions oftoluene and benzene, respectively.

filled reactant, it is ascertained that there is no reactantadsorption on the wall inside the mixing tank. Furtherdetails of the present rapid compression machine andexperimental procedure can be found in [19,20].

Autoignition investigations for toluene/O2/N2/Arand benzene/O2/N2/Ar mixtures are conducted overthe temperature range of 920–1100 K, pressure rangeof 25–45 bar, and equivalence ratio range of 0.5–1.0. To isolate the chemical effects, the same specificheat ratio is maintained for all of the mixtures in-vestigated by adjusting the relative proportion of N2and Ar in the diluent. Table 1 lists the compositionsof gas mixtures prepared herein, by specifying thepartial pressures of individual constituents. The cor-responding equivalence ratio (φ) and fuel mole frac-tion of each mixture are also indicated in Table 1.Toluene and benzene employed are, respectively, 99.9and 99.5% pure (Fisher Scientific), and all the gasesfrom Praxair are of ultra high purity—O2, 99.993%;N2, 99.999%; and Ar, 99.995%. For a given mixturecomposition with initial temperature of room temper-ature, the compressed gas temperature at the end ofcompression (top dead center; TDC), TC , is variedby altering the compression ratio, whereas the desiredpressure at TDC is obtained by varying the initialpressure of the reacting mixture for a given compres-sion ratio. The temperature at TDC is determined bythe adiabatic core hypothesis according to the relation

TC∫

T0

γ

γ − 1

dT

T= ln[PC/P0],

where P0 is the initial pressure, T0 is the initial tem-perature, γ is the specific heat ratio that is a functionof temperature, and PC is the measured pressure atTDC.

Numerical modeling of ignition delay is per-formed using the Sandia SENKIN code [21] in con-junction with CHEMKIN [22]. The modeling begins

G. Mittal, C.-J. Sung / Combustion and Flame 150 (2007) 355–368 357

from the start of compression stroke and includes theeffect of heat loss during the compression and afterthe end of compression. While the description of thepresent heat transfer model is detailed in [20], a dis-cussion of the model validation is included as an ap-pendix. Experimental results are compared with sim-ulations using various detailed mechanisms availablein the literature, including the mechanisms of Klotzet al. [9], Dagaut et al. [11], Bounaceur et al. [14],Sivaramakrishnan et al. [16], Djurisic et al. [23], andPitz et al. [24]. Moreover, the mechanisms of Klotz etal. [9], Sivaramakrishnan et al. [16], and Djurisic etal. [23] will be respectively referred to as KBG, STB,and DJW in the following.

The KBG mechanism [9] was primarily validatedagainst atmospheric pressure flow reactor species pro-files of toluene–butane blends at around 1200 K. Themechanism of Dagaut et al. [11] has been validatedagainst oxidation data of toluene in a jet stirred reac-tor (JSR) at 1 atm and 1000–1375 K, oxidation dataof benzene in a JSR up to 10 atm, high-temperatureshock tube ignition delay measurements for benzeneand toluene, and atmospheric pressure laminar flamespeeds of benzene and toluene. The mechanism ofBounaceur et al. [14] was validated against tolueneoxidation in a JSR at atmospheric pressure and around900 K, shock tube ignition delay measurements upto 8.7 bar, species profile from high-pressure shocktube at 613 bar, and atmospheric pressure flow reac-tor data around 1200 K. The STB mechanism [16]was primarily validated against toluene oxidation datain a high-pressure shock tube at pressures from 25to 610 bar, temperatures from 1200 to 1500 K, andequivalence ratios ranging from 1 to 5. It also predictsreasonably well the shock tube ignition delay mea-

surements at around 3 atm and flow reactor concentra-tion profile of toluene at 1 atm and 1188 K. The DJWmechanism [23] was validated against shock tube ig-nition delay measurements for benzene and toluene atpressures up to 7.4 atm, laminar flame speeds of ben-zene and toluene, low-pressure flame structure of pre-mixed benzene flame, and flow reactor species pro-files for benzene and toluene oxidation at 1 atm andaround 1200 K. The mechanism of Pitz et al. [24] fortoluene was developed using shock tube ignition de-lay measurements, atmospheric pressure flow reactorspecies profiles, and extinction and ignition data instrained nonpremixed flames. From the above discus-sion, it is clear that most previous model developmentand validation has been limited to the conditions ofhigh temperatures and low pressures, because litera-ture data for toluene and benzene under conditions oflow temperatures (<1100 K) and elevated pressures(>10 bar) are meager. Therefore, the availability ofexperimental data of high fidelity over broad rangesof pressures and temperatures will greatly help scru-tinize the comprehensiveness of a chemical kineticmechanism.

3. Results and discussion

Fig. 1 shows an example of pressure traces for theignition of a toluene/O2/N2/Ar mixture (0.5/6/9.32/43.61 by mole) with an initial temperature of 298.8 Kand an initial pressure of 872 Torr. The measuredpressure and the deduced temperature at the end ofcompression (t = 0) are PC = 45 bar and TC =1019 K, respectively. Overlapping pressure traces offive experiments conducted under similar conditions

Fig. 1. Pressure traces illustrating the ignition and repeatability of five experiments. Molar composition: toluene/O2/N2/Ar= 0.5/6/9.32/43.61. Initial conditions: T0 = 298.8 K, P0 = 872 Torr, φ = 0.75. Conditions at TDC: TC = 1019 K and PC =45 bar.


are also compared in Fig. 1, demonstrating that allthe pressure traces closely follow each other and thepresent experimental data are highly repeatable. It isalso seen from Fig. 1 that the compression stroke is∼25 ms, and the ignition delay (τ ) is defined as thetime from the end of the compression stroke, wherethe pressure peaks at t = 0, to the instant of 50% ofthe total pressure rise due to ignition. Because of therapid rise in pressure during ignition, other definitionsof ignition delay, such as the inflection point in thepressure history, yield similar results.

A detailed uncertainty analysis is conducted to de-termine the uncertainty in the reported ignition delaysas functions of PC , TC , and reactant composition, fol-lowing the uncertainty propagation approach of [25].It is noted that the uncertainty in the measured igni-tion delay arises from different sources, including theuncertainty in preparing the initial reactant concen-trations due to the errors associated with partial pres-sure measurements, the uncertainty in the measuredpressure history due to the accuracy of the piezoelec-tric transducer, and the uncertainty in the deducedcompressed gas temperature. According to the manu-facturer’s specifications, the uncertainty in the initialtemperature measurement is ±0.5 K and the accu-racy of the piezoelectric transducer is 0.5%. As such,the uncertainty of the deduced TC based on the adia-batic core hypothesis is typically ±3 K. We will showin due course the ignition delay time correlations forboth toluene and benzene under the conditions inves-tigated. The resulting correlations are subsequentlyused, along with the estimated uncertainties associ-ated with various parameters in the correlation, todetermine the uncertainty in the reported ignition de-lays. The overall uncertainty in the present ignitiondelay measurements is estimated to be in the range of7.5–11%.

Fig. 2 illustrates an example of the modeling ofthe present RCM experiments. Experimental and sim-ulated pressure traces for both the reactive mixtureand the corresponding nonreactive mixture with thesame specific heat ratio are shown in Fig. 2. The non-reactive mixture is composed of inert gases of N2and Ar and is prepared with the same initial pressureand initial temperature as for the reactive mixture.The experimental and computed pressure traces fornonreactive mixtures are seen to match very well forboth compression and postcompression events, indi-cating the adequacy of the modeling adopted herein.As such, the parameters required for the heat trans-fer model can be determined from the pressure traceof the nonreactive experiment. The empirically de-termined parameters are then used for simulating thecorresponding reactive case [20]. For simulating thereactive mixture in Fig. 2, the DJW mechanism [23]is employed. Comparing to the experimental data, the

Fig. 2. Experimental and simulated pressure traces fortoluene autoignition. Both reactive and nonreactive condi-tions are shown. Molar composition for the reactive mix-ture: toluene/O2/N2/Ar = 0.5/4.5/5.25/41.75. Conditions atTDC: TC = 1045 K and PC = 44.6 bar. The mechanism ofDjurisic et al. [23] is used in the simulation.

simulated pressure trace for reactive mixture showsdifferent behaviors of heat release and ignition delay.Therefore, the difference in pressure traces for the re-active mixture and the discrepancy in ignition delaycan be used to assess the predictability of the mecha-nism employed.

3.1. Ignition delay

Fig. 3 presents the measured ignition delay timesfor stoichiometric toluene/O2/inert mixtures at a pres-sure near 45 bar at TDC as a function of the in-verse of compressed gas temperature, TC . A fairlylinear variation is seen in this semi-log plot. It hasto be noted that the experimental data with ignition

Fig. 3. Comparison of experimental and computed ignitiondelays for stoichiometric toluene/O2/inert mixtures. Toluenemole fraction, Xtoluene, is 0.00962. PC = 45 bar.


Fig. 4. Effect of equivalence ratio on ignition delay.Xtoluene = 0.00962 and PC = 45 bar. Equivalence ratio isvaried by varying the mole fraction of O2.

delays longer than 100 ms could be influenced bythe increased nonhomogeneity within the combustionchamber and hence should be taken as an indication ofa general trend. The calculated ignition delays usingvarious kinetic mechanisms are also shown and com-pared in Fig. 3. Significant differences between theexperimental data and the simulations can be noted.The mechanisms of Dagaut et al. [11] and Bounaceuret al. [14] yield ignition delays larger by an order ofmagnitude than the experimental data, whereas that ofPitz et al. [24] gives significantly faster ignition. Onthe other hand, the STB mechanism [16] predicts sig-nificantly lower activation energy (i.e., smaller slopevalue in Fig. 3), resulting in overpredictions at hightemperatures and underpredictions at low tempera-tures. While simulations using the KBG mechanism[9] and the DJW mechanism [23] perform relativelybetter with respect to the trend and activation energy,the predicted ignition delays are much shorter. ForPC = 25 bar, comparison of the experimental igni-tion delay data and the computed results using variousmechanisms shows the similar degree of discrepancyas observed at 45 bar in Fig. 3.

Fig. 4 presents the effect of change in equivalenceratio (φ) on ignition delay at PC = 45 bar. These ex-periments are conducted by keeping the toluene molefraction, Xtoluene, constant at 0.00962, and equiva-lence ratio is changed by varying the mole fractionof O2 through the composition variation of the in-ert gases. It is seen from Fig. 4 that, for a given TC ,ignition delay decreases significantly with increasingO2 addition or decreasing equivalence ratio. Fig. 5presents similar results at a relatively lower com-pressed pressure of 25 bar.

Fig. 6 compares the present RCM data withthe shock tube results of Davidson et al. [18] fortoluene/oxidizer mixtures of φ = 0.5. In the experi-

Fig. 5. Effect of equivalence ratio on ignition delay.Xtoluene = 0.00962 and PC = 25 bar. Equivalence ratio isvaried by varying the mole fraction of O2.

Fig. 6. Comparison of the present experimental data (filledsymbols) with the previous results from Davidson et al. [18](lines and open symbols) for toluene/oxidizer mixtures ofφ = 0.5. The data of Davidson et al. [18] have been normal-ized to 25 and 45 bar using their fitted correlation.

ments of Davidson et al. [18], Xtoluene was 0.0115,whereas a slightly lower but comparable value of0.00962 is studied in the present experiments. Fur-thermore, in Fig. 6 the data of [18] have been nor-malized to 25 and 45 bar using their fitted correlation,while the RCM data are plotted by taking the pressureand temperature at TDC as the reference. Overall, thetwo datasets appear to be consistent in that they agreereasonably well in the overlapped conditions. How-ever, it is seen from Fig. 6 that the present RCM datain the low temperature regime have a steeper slope,which could be due to the effect of heat loss, espe-cially for the conditions with relatively long ignitiondelay time. Similar comparison between the presentRCM data and the results of [18] for the stoichiomet-ric mixtures cannot be readily conducted because oftheir significant difference in Xtoluene.

Fig. 7 shows the effect of equivalence ratio on ig-nition delay by keeping PC and TC constant, along


Fig. 7. Experimental (symbols) and simulated (lines) re-sults for the effect of equivalence ratio on ignition delayof toluene. Xtoluene = 0.00962, PC = 45 bar, and TC =1045 K.

Fig. 8. Experimental results for the effect of toluene molefraction on ignition delay for stoichiometric toluene/O2/inertmixtures. PC = 25 bar (upper pair) and 45 bar (lower pair).

with simulated results by using various mechanisms.The results using the mechanisms of Dagaut et al.[11] and Bounaceur et al. [14] are not shown in Fig. 7due to very long computed ignition delay. For all themechanisms considered in Fig. 7, the calculated val-ues are seen to follow the same trend as the experi-mental data: oxygen addition reduces ignition delay.However, the STB mechanism [16] predicts strongereffect of O2 addition than that observed in the ex-periments. It is also of interest to note that the DJWmechanism [23] predicts linear variation of ignitiondelay with equivalence ratio.

The effect of fuel mole fraction on ignition de-lay is shown in Fig. 8. Stoichiometric mixtures aretested herein at two different compressed pressures,PC = 25 and 45 bar. Ignition delay is seen to drop

Fig. 9. Experimental (symbols) and simulated (lines) resultsfor the effect of fuel mole fraction on ignition delay for sto-ichiometric toluene/O2/inert mixtures. Conditions at TDC:TC = 1045 K and PC = 45 bar.

Fig. 10. Comparison of experimental and computed ignitiondelays for stoichiometric benzene/O2/inert mixtures. Ben-zene mole fraction, Xbenzene, is 0.00962. PC = 45 bar.

significantly at a higher fuel loading. In Fig. 9, all themechanisms considered predict this reduction in ig-nition delay with increasing fuel loading. However,the predicted reduction by the STB mechanism [16]is somewhat less than the experimental observation.

Fig. 10 compares the measured ignition delays forstoichiometric benzene/O2/inert mixtures with sim-ulations at PC = 45 bar. Here, the benzene molefraction, Xbenzene, is kept constant at 0.00962. Liketoluene, in the case of benzene significant discrepancyamong mechanisms is also noted. The mechanism ofBounaceur et al. [14] results in very long ignition de-lay and hence is not shown in Fig. 10.

Fig. 11 compares the measured ignition delaysfor benzene and toluene. It is noted that under thepresent experimental conditions, toluene autoignitionis more rapid than benzene autoignition. Moreover,in contrast to toluene, a best linear fit to the igni-


Fig. 11. Comparison of experimental ignition delays for ben-zene and toluene.

tion delay data of benzene in Fig. 11 yields a steeperslope. Therefore, at higher compressed temperatures(TC > 1200 K), benzene is expected to autoignite ear-lier than toluene. The observation at the temperaturerange investigated is consistent with the discussion in[26] and is due to the presence of labile methyl C–Hbond in toluene, which is broken relatively easily atlow temperatures.

A regression analysis is further conducted for thepresent ignition delay data of toluene and benzene,yielding the following two correlations:

τtoluene = 1.4 × 10−12P−2C

(Xtoluene)−0.8(XO2)

−1.6

× exp(30759/TC)

with R2 = 0.984

and

τbenzene = 8.83 × 10−25(XO2)−1.22 exp(63281/TC)

with R2 = 0.983,

where τtoluene and τbenzene are in ms, PC is in bar,TC is in Kelvin, XO2 is the initial mole fraction ofO2, and R2 is the multiple regression correlation co-efficient. We also note that the correlation includesall of the experimental data with ignition delay lessthan 100 ms. In addition, since the present limited ex-perimental data of benzene are conducted by keepingboth PC and Xbenzene constant, the resulting corre-lation does not exhibit the dependence of τbenzene onPC and Xbenzene. Fig. 12 shows the experimental datanormalized to PC = 45 bar, Xfuel = 0.00962, andφ = 1 (the corresponding values of XO2 are 0.08658and 0.07215 for toluene and benzene, respectively)along with the fitted correlations. It is seen fromFig. 12 that the correlations adequately represent thepresent experimental data. Following the uncertaintypropagation analysis of [25] and considering the esti-mated uncertainties of various parameters involved in

Fig. 12. Experimental ignition delay data (symbols) fortoluene and benzene normalized to PC = 45 bar, Xfuel =0.00962, and φ = 1. The fitted correlations are shown assolid lines.

the correlation, as discussed earlier, the overall uncer-tainty in the ignition delays reported herein is foundto be around ±9%, which is primarily due to the un-certainty in the deduced compressed temperature.

3.2. Preignition heat release

In a rapid compression machine, the preignitionpressure rise is related to the energy release before ig-nition. Fig. 13 shows the experimental pressure tracesobtained using stoichiometric toluene mixtures at apressure of 45 bar at TDC for different compressedtemperatures. In Fig. 13, for the temperature range of1000–1100 K, the experimental pressure–time tracesshow small amount of preignition heat release fol-lowed by rapid ignition. These results are very dif-ferent from the recent investigation by Davidson etal. [18] in a shock tube under comparable conditionsof temperature and pressure. In the experiments ofDavidson et al. [18], shown as an inset in Fig. 13, theirpressure profiles show an induction time followed bya smooth rise to an intermediate plateau, with 33%rise in pressure, before the final ignition. At relativelylower temperatures (∼1000 K), Davidson et al. [18]noted that the rise of pressure is initially slow andsubsequently linear before final ignition, as seen inthe inset of Fig. 13.

Incidentally, we notice that a trend similar to thatin [18], namely significant, nearly linear pressure risebefore ignition, would occur when the combustionchamber becomes contaminated after a number ofsuccessive runs. Specifically, when the combustionchamber is cleaned with a solvent after a number ofruns, some blackish deposits on the inner wall surfaceare noted. An example of such pressure traces show-ing the nearly linear preignition pressure rise and theeffect of chamber surface contamination on ignition


Fig. 13. Experimental pressure traces showing the preignition heat release. Molar composition: toluene/O2/N2/Ar = 0.5/4.5/5.25/41.75. Xtoluene = 0.00962 and PC = 45 bar. The inset shows the pressure traces of the shock tube experiments ofDavidson et al. [18] for stoichiometric toluene/air mixtures with Xtoluene = 0.0228.

Fig. 14. Experimental pressure traces showing the preignition heat release with a vitiated reaction chamber. It also demonstratesthat when the reaction chamber is contaminated, the experiment is no longer reproducible. Molar composition: toluene/O2/N2/Ar= 0.5/4.5/5.25/41.75. Xtoluene = 0.00962 and TC = 990 K.

delay in RCM is shown in Fig. 14. Fig. 14 com-pares the experimental results of two consecutive runswith the same condition, demonstrating that once thereaction chamber is contaminated the experiment isno longer reproducible. However, when the combus-tion chamber is carefully purged and cleaned, ignitionis observed to occur without significant preignitionpressure rise, as shown in Fig. 13, and is highly re-producible, as demonstrated in Fig. 1. It is noted thatthe recent shock tube experiments of Wang et al. [27]using H2/air/steam mixtures also showed that the ig-nition process is very sensitive to the inner surfacequality of the tube. Wang et al. [27] further demon-strated that, if the tube is contaminated, their mea-sured ignition delay time cannot be reproduced. It hasto be pointed out that all the experimental data pre-

sented herein, except Fig. 14, are obtained without theeffect of chamber surface contamination.

Furthermore, while the experimental data shownin Fig. 13 are at a lower fuel loading (Xtoluene =0.00962) than the experiments of Davidson et al. [18](Xtoluene = 0.0228), we obtain similar results as inFig. 13 at a higher fuel mole fraction of 0.013. There-fore, the differences in preignition heat release be-tween the present study and [18] are not expectedto be caused by different fuel loadings. Further in-dependent investigations are needed to resolve sucha discrepancy between the present RCM experimentsand that observed by Davidson et al. [18].

Fig. 15 compares the experimental pressure tracewith simulations at a compressed pressure near 45 bar,revealing the features of preignition heat release. The


Fig. 15. Experimental and simulated pressure traces showing preignition heat release for stoichiometric conditions. (a) Tolueneautoignition, Xtoluene = 0.00962, TC = 1045 K, and PC = 44.6 bar; (b) benzene autoignition, Xbenzene = 0.00962, TC =1102 K, and PC = 45.1 bar.

compressed temperatures are 1045 and 1102 K fortoluene and benzene, respectively. Experimental pres-sure record for the corresponding inert gas mixtureis also included for reference. In Fig. 15, apart fromquantitative differences in ignition delay, the simu-lated pressure records demonstrate behaviors of heatrelease different from those of the experiment. InFig. 15a, the DJW mechanism [23] and the KBGmechanism [9] show higher heat release before igni-tion, whereas the STB mechanism [16] shows nearlylinear preignition heat release. In the computed profileusing the STB mechanism [16], an inflection point isalso noted, which becomes more pronounced at lowertemperatures. Similar discrepancies are also seen inthe preignition heat release for benzene, as shown inFig. 15b. Another interesting feature is that, for bothtoluene and benzene, the mechanism of Pitz et al.[24] shows very less preignition pressure rise, whichis consistent with the experimental observation. To

identify the reasons for this behavior in heat release,flux analysis and heat release analysis are conducted.

3.3. Kinetic analysis

Fig. 16 identifies the dominant reaction path-ways for toluene autoignition under the condition ofFig. 15a through integrated flow analysis using theSTB [16] and DJW [23] mechanisms. For the STBmechanism, reaction flux is integrated up to the inflec-tion point in the heat release profile, and for the DJWmechanism it is integrated up to the time when tem-perature is increased to the same value as that at theinflection point of STB. Therefore, the integrated fluxgives an indication of dominant reactions before igni-tion takes place. In Fig. 16, the numbers and speciesalong the arrows represent the percentage consump-tion of the corresponding starting reactant through thespecific reaction with the indicated species. More-


Fig. 16. Integrated flow analysis for toluene autoignition, from TDC to a reference time prior to ignition, based on Fig. 15a.Mechanisms used: Sivaramakrishnan et al. [16] (shown outside the parentheses) and Djurisic et al. [23] (shown inside theparentheses). The numbers and species along the arrows represent the percentage consumptions of reactant through the specificreaction with that species.

over, the percentage consumptions obtained fromthe DJW and the STB mechanisms are shown in-side and outside the parentheses, respectively. Fromboth mechanisms, similarity in reaction pathwayscan be noted. The overall reaction proceeds primar-ily through C6H5CH3 → C6H5CH2 → C6H5CHO→ C6H5CO → C6H5 → C6H5O → C6H5OH, fol-lowed by the ring-breaking reactions. The reactionof C6H5CH3 + OH is seen to account for the majorconsumption of toluene during the induction period.In addition, there is significant formation of cresol(HOC6H4CH3) from toluene. Integrated flow analy-sis is also conducted using the mechanisms of Dagautet al. [11] and Bounaceur et al. [14], mostly showingpathways similar to those in Fig. 16, with the majordifference being the formation of significant amountof dibenzyl (C14H14). Furthermore, the present ex-perimental conditions of elevated pressure and low-to-intermediate temperature highlight the importanceof reactions involving HO2 radical, which do not typ-

ically assume prominence at low pressures and hightemperatures.

To elucidate the differences in the simulated re-sults from these mechanisms, Fig. 17 shows the pro-files of selected species and OH flux for the condi-tion of Fig. 15a. In Fig. 17, for clarity some majorspecies such as CO, CO2, and H2O are not shown.From both mechanisms, the dominant species prior toignition include cresol, phenol (C6H5OH), benzalde-hyde (C6H5CHO), and quinone (C6H4O2). However,it is seen for the STB mechanism that before ignitionthe concentrations of intermediate hydrocarbons be-come larger than that of toluene due to fast depletionof toluene. Significant differences can be also seen inOH flux from two mechanisms. The reason for this isdiscussed below.

In the STB mechanism [16], the rate constant forC6H5CH2 + HO2 → C6H5CHO + H + OH [R1]was significantly increased (by a factor of 70) fromthe base mechanism of Klotz et al. [9]. As a re-


Fig. 17. Computed species profiles and OH fluxes for toluene autoignition based on Fig. 15a. Mechanisms used: Djurisic et al.[23] (left) and Sivaramakrishnan et al. [16] (right).

sult, there is significant amount of toluene consump-tion and heat release through C6H5CH3 + OH →C6H5CH2 + H2O [R2], followed by [R1]. The sub-sequent reaction of H radical, H + O2 (+M) → HO2(+M), is also highly exothermic. Moreover, the OHradicals formed from [R1] react with toluene via [R2]and with benzaldehyde through C6H5CHO + OH →C6H5CO + H2O, followed by benzoyl (C6H5CO)decomposition to phenyl (C6H5) and CO. Phenyl sub-sequently forms phenoxy (C6H5O) that reacts with Hradical through C6H5O + H → C6H5OH. In addi-tion, significant amount of toluene also reacts with Oatom to form cresoxy (OC6H4CH3) and cresol. Thesereactions lead to significant amount of heat releasewithout any chain branching. Initially, the majority ofthe consumption of OH radicals is through toluene.However, if the rate constant of [R1] is increasedsubstantially, the concentrations of benzaldehyde andother intermediaries become significantly large andthat of toluene significantly less even before ignitiontakes place. Furthermore, since the reaction rate ofC6H5CHO + OH is about 2.5 times the reaction of

C6H5CH3 + OH, the reaction of OH and benzalde-hyde becomes predominant. This in turns changes theconsumption path of OH and slows the consumptionof toluene prior to ignition. As such, both the over-all production of OH and the preignition heat releasereduce and an inflection point in the heat release pro-file is observed when using the STB mechanism [16].From this analysis it appears that the increased rate ofC6H5CH2 + HO2 → C6H5CHO + H + OH in [16]to match the shock tube data leads to the peculiar heatrelease profile in simulation, which is not seen in thepresent experiments.

It is further noted that a recent study [28] reportedthat during autoignition of toluene in a rapid compres-sion machine, no intermediate species were detectedprior to ignition and the increase of pressure at theignition point was very rapid with benzaldehyde andbenzene being detected only at the onset of ignition.The present experiments are consistent with the ob-servation in [28] in that the preignition heat releaseand the consumption of toluene before ignition are notsubstantial. Furthermore, since rapid ignition occurs


Fig. 18. Sensitivity analysis for ignition delay of stoichiometric toluene mixture. PC = 45 bar, TC = 1045 bar, and Xtoluene =0.00962.

due to the decomposition of the degenerate chain-branching agent, H2O2, which is formed from the Habstraction reactions by HO2, it is possible that thediscrepancies in the simulations are because of thereactions involving the formation of HO2 and its sub-sequent consumption to form primarily H2O2, insteadof HO2 reacting with toluene-related radicals such asbenzyl (C6H5CH2).

Fig. 18 shows the results of sensitivity analysisby using various mechanisms. For this, the rate con-stant of each reaction, ki , is individually doubled.The resulting percentage change in the ignition delay,namely 100 × [τ (2ki) − τ (ki)]/τ(ki), is then takenas the percentage sensitivity of that particular reac-tion. All reactions with percentage sensitivity greaterthan 5% are included in Fig. 18. Due to the major dif-ferences in some key reactions, vastly different sensi-tivities (magnitude and sign) are noted from variousmechanisms.

4. Concluding remarks

Autoignition of toluene and benzene is investi-gated in a rapid compression machine at pressuresfrom 25 to 45 bar and temperatures from 920 to1100 K. Effects of oxygen concentration and fuelloading on autoignition are also investigated. Theexperimental data obtained are of direct relevancefor validation of kinetic models of toluene and ben-zene at conditions of high pressure and low-to-intermediate temperature. Major discrepancies arenoted in the predictions of various existing kineticmechanisms with respect to ignition delay and preig-nition heat release. Further experimental characteri-zation of species evolution during high-pressure au-toignition will be needed for more rigorous validationand development of comprehensive reaction mecha-nisms. So far, the development of a comprehensivekinetic model for toluene and benzene has proven to


Fig. 19. Comparison of CFD-determined RCM results with the deduced postcompression temperature histories using two differ-ent approaches: (1) including a heat loss term in the energy equation and (2) using effective volume of core region.

be challenging. Improved elucidation of the key reac-tions involving HO2 and other sensitive reactions canhelp resolve the issues.

Acknowledgments

This work has been supported by the National Sci-ence Foundation under Grant No. 0133161 and by theArmy Research Office under Grant No. W911NF-06-1-0155. The authors thank Professor H. Wang at Uni-versity of Southern California, Professor K. Brezin-sky and Dr. R. Sivaramakrishnan at University of Illi-nois at Chicago, Dr. P. Dagaut and Dr. R. Bounaceurat the Centre National de la Recherche Scientifique,and Dr. W.J. Pitz at the Lawrence Livermore Na-tional Laboratory for providing their detailed reactionmechanisms. We also acknowledge Dr. D.F. Davidsonfor sharing the shock tube pressure data that appearedin [18].

Appendix A. Discussion of RCM heat lossmodeling

In an RCM, pressure decreases during the post-compression period due to heat loss through the wallsof the combustion chamber. Consequently, a numeri-cal model which accounts for the effect of heat lossis needed to simulate the RCM experimental data.There are typically two approaches in considering theeffect of heat loss after reaching the TDC in RCMmodeling. One approach is to include a heat lossterm in the energy equation while keeping the reac-tor volume constant (e.g., [29,30]), and the other is

to specify a volume expansion term while applyingthe adiabatic core assumption (e.g., [20,31,32]). Forthe former approach, an empirical parameter in theheat loss term is used and varied to match the sim-ulated pressure trace with the experimental data. Therationale behind the latter approach is that as the near-wall boundary layer cools during the postcompressionperiod, the core gas away from the boundary layerexperiences an expansion even though the geomet-ric volume of the combustion chamber remains thesame after reaching the TDC. Especially, when theroll-up vortex is effectively suppressed (e.g., by us-ing a creviced piston), the effect of heat loss reachesthe core gas only through this expansion. Therefore,the experimental pressure trace can be used to backcalculate the “effective volume” of the core region,which is time dependent. The heat loss model devel-oped for the present RCM is based on the approachof “effective volume” and has been documented else-where [20,32].

Both of the above-mentioned approaches can beused to simulate and match the experimental pres-sure history. However, the two approaches lead todifferent temperature histories during the postcom-pression period. In the following, based on a nonre-active RCM run with N2 gas, we compare the result-ing postcompression temperature profiles using thetwo different approaches. The baseline pressure andtemperature histories during compression and post-compression event are obtained from a computationalfluid dynamic (CFD) simulation using the STAR-CDpackage. The simulation details and the configura-tion of the creviced piston head employed have beendocumented in [20]. The initial conditions specifiedin the simulations are N2 at rest with temperature


of 297 K and pressure of 260 Torr. Fig. 19 showsthe time evolution of pressure and maximum instan-taneous temperature in the chamber obtained fromthe CFD calculations, with time = 0 representing theTDC. Again, both pressure and maximum tempera-ture decrease with time during the postcompressionperiod because of heat loss.

To highlight the difference in temperature histo-ries deduced from the two approaches, we further as-sume that both approaches predict the same PC andTC and match the postcompression pressure trace ob-tained from CFD. For the approach of including aheat loss term in the energy equation, the ideal gaslaw is applied to determine the postcompression tem-perature based on T (t) = TC [P(t)/PC ] due to con-stant reactor volume. With the approach of “effec-tive volume,” the time-dependent temperature profilecan be deduced using the adiabatic core hypothesis,T (t) = TC [P(t)/PC ](γ−1)/γ , or the ideal gas law,T (t) = TC [P(t)/PC ][V (t)/V (0)], where V (t) is thetime-varying effective volume. Fig. 19 also comparesthe two deduced temperature histories, showing thatthe approach of including a heat loss term leads toa much lower temperature history. This is becausein this approach heat loss is implicitly assumed tobe throughout the entire volume of the combustionchamber and not just within the boundary layer. Fur-thermore, it is seen from Fig. 19 that the tempera-ture deduced from the approach of “effective volume”matches exactly with the maximum temperature ob-tained from the CFD for long duration. Through thisexample we computationally validate the approach ofusing “effective volume” for simulating the presentRCM experiments.

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Autoignition of toluene and benzene at elevated pressures in a rapid compression machine

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