Combustion and Flame 168 (2016) 216–227

Contents lists available at ScienceDirect

Combustion and Flame

journal homepage: www.elsevier.com/locate/combustflame

Experimental study of 2,5-dimethylfuran and 2-methylfuran in a rapid

compression machine: Comparison of the ignition delay times and

reactivity at low to intermediate temperature

Nan Xu

a , Yingtao Wu

a , Chenglong Tang

a , ∗, Peng Zhang

b , Xin He

c , Zhi Wang

b , Zuohua Huang

a , ∗

a State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an 710049, China b State Key Laboratory of Automotive Safety and Energy, Tsinghua University, Beijing 10 0 084, China c Aramco Services Company: Aramco Research Center-Detroit, Novi, MI 48377, USA

a r t i c l e i n f o

Article history:

Received 29 October 2015

Revised 16 March 2016

Accepted 16 March 2016

Available online 7 April 2016

Keywords:

Low temperature ignition

Rapid compression machine

2,5-Dimethylfuran

2-Methylfuran

a b s t r a c t

In this work, low to intermediate temperature autoignition behaviors of DMF (2,5-dimethylfuran) were

investigated at pressures of 16 and 30 bar, temperatures from 737 to 1143 K, and equivalence ratios of

0.5, 0.75, 1.0, and 2.0 for different fuel concentrations using a rapid compression machine. Ignition de-

lay times of 1% MF (2-methylfuran) stoichiometric mixture was measured at 16 and 30 bar in order to

compare its combustion chemistry with DMF. Two mechanisms were employed to predict experimental

ignition data. The mechanism of Somers et al. (2013, 2014) shows reasonable agreement with experimen-

tal results when some modifications are incorporated. Both experiments and numerical simulations show

that the reactivity of DMF and MF is similar in the low to intermediate temperature range. However, the

ignition delay time of MF is slightly more sensitive to temperature, compared to DMF, and a crossover

temperature T c 2 was observed, below which DMF preserves a slightly higher reactivity. Kinetic analysis by

examining the reaction flux and the sensitivity coefficients at temperatures close to T c 2 were conducted

to interpret the dominant reaction kinetics of the ignition delay times of DMF and MF. Finally, compar-

isons between the ignition delay times of DMF and PRF (primary reference fuel) components ( iso -octane,

toluene, and n -heptane) was also conducted due to the interests of its practical applications, based on

PRF ignition data in literature.

© 2016 The Combustion Institute. Published by Elsevier Inc. All rights reserved.

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1. Introduction

Depletion of fossil fuels and air pollution caused by fossil fu-

els combustion raise more public concerns over the application

of alternative fuels to replace gasoline. DMF (2,5-dimethylfuran),

which have been reported recently to be produced from non-food

biomass [1–5] is one of the most promising alternative fuel can-

didates. Similar to DMF, MF (2-methylfuran) is also a second gen-

eration biofuel that can be produced from lignocellulosic biomass

[6, 7] . In addition, the energy density and RON (Research Octane

Number) of DMF and MF are close to those of commercial gaso-

line, which renders its application in internal combustion engines

more attractive, either by SI (spark ignition) or CI (compression ig-

nition) engines.

∗ Corresponding authors. Fax: + 86 29 82668789.

E-mail addresses: chenglongtang@mail.xjtu.edu.cn (C. Tang),

zhhuang@mail.xjtu.edu.cn (Z. Huang).

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http://dx.doi.org/10.1016/j.combustflame.2016.03.016

0010-2180/© 2016 The Combustion Institute. Published by Elsevier Inc. All rights reserved

Studies on performance and emissions of DMF fueled SI en-

ines have been extensively reported. Zhong et al. [8] compared

he combustion characteristics of DMF, gasoline, and ethanol in a

ingle cylinder SI engine. Daniel et al. [9–11] studied the effect of

perating parameters on the SI engine performance of DMF, gaso-

ine, and ethanol. Studies on MF fueled spark ignition engine per-

ormance have been conducted by Wang et al. [12] recently. Their

esults indicate that the combustion performance of DMF and MF

s similar with that of gasoline, and better emissions are observed

xcept for NOx. For CI engines, Zhang et al. [13] and Chen et al.

14] compared the performance of DMF/diesel blend and gaso-

ine/diesel blend. Their results show that the addition of DMF pro-

ongs ignition delay times and results in higher NOx emissions, and

he engine need to be run on a higher EGR (exhaust gas recircula-

ion) ratio.

The above practical engine investigations have demonstrated

he flexibility of DMF and MF in conventional engines. HCCI (ho-

ogeneous charge compression ignition) mode maintains the mer-

ts of high combustion efficiency and low NOx and PM emissions

.

N. Xu et al. / Combustion and Flame 168 (2016) 216–227 217

Table 1

Alkylfuran ignition delay time studies in literature using shock tubes.

Fuel Mixture φ P (bar) T (K) Ref.

DMF Diluted 0.25–1.0 1 and 4 1300–1831 [26]

DMF Diluted 0.5–2.0 1 1350–1800 [28]

DMF Diluted 1.0 20 and 80 820–1210 [28]

MF Diluted 0.5–2.0 1 120 0 180 0 [27]

MF Diluted 0.25–2.0 1.25–10.65 1120–1700 [32]

DMF and MF In air 0.5–2.0 2–12 977–1570 [29]

MF In air 1.0 40 780–1100 [33]

DMF and DMF/

iso -octane

In air 0.5–2.0 5–12 1009–1392 [30]

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Table 2

Test mixture composition.

Fuel φ Fuel concentration P (bar)

DMF 0.5, 0.75, 1.0, 2.0 1% 16 and 30

1.0 0.5% and 2.0% 16

MF 1.0 1% 16 and 30

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15–17] . The understanding of the chemical kinetics of alkylfurans

ould help meet economic and emission legislations in the future

nd facilitate their application in HCCI combustion as there is no

xtra method to control auto-ignition.

Extensive fundamental studies emphasizing on the chemical ki-

etics of alkylfurans have been conducted, and we only give a

rief review on related works. A consecutive low pressure flame

tudies on laminar premixed DMF, MF, and furan/O 2 /Ar mixtures

ave been conducted by Liu et al. [18] , Togbé et al. [19] , and Tran

t al. [20] . Relevant kinetic mechanism was constructed, and the

ffects of methyl branches on the flame structures were then inter-

reted. Wu et al. [21–24] conducted systematic study on the lam-

nar flame propagation characteristics of DMF/air mixtures using a

onstant volume combustion chamber. Tian et al. [25] concluded

hat the difference of DMF and gasoline laminar burning veloci-

ies is within 10% when equivalence ratio falls in 0.9–1.1. With re-

ard to ignition delay times. The low pressure ignition delay times

f DMF were measured by Sirjean et al. [26] and used to validate

heir newly constructed DMF mechanism. Somers et al. [27] mea-

ured atmospheric ignition delay times of MF, and the ignition de-

ay times of DMF at atmospheric pressure and high pressures (20

nd 80 bar) were also provided [28] . A kinetic mechanism has been

onstructed and validated against species profiles, laminar flame

peeds, and ignition delay times, which shows fairly good agree-

ents. Air diluted ignition delay times of alkylfurans have been

easured by Eldeeb et al. [29,30] . Among the three alkylfurans,

F shows a higher reactivity than DMF, and the reactivity of fu-

an is the lowest. Moreover, DMF is relatively unreactive compared

ith iso -octane under their circumstances, and the newly com-

ined mechanism of iso -octane (Mehl et al. [31] ) and DMF (Somers

t al. [28] ) could give good agreement with ignition delay times of

oth fuels.

Table 1 summarizes the related fundamental combustion study

n DMF and MF. Compared with the high temperature kinetics

tudy of DMF, the low to intermediate temperature research to-

ards DMF auto-ignition characteristics is relatively insufficiently

xplored, in terms of experimental data and kinetic modeling,

hile the low to intermediate is temperature range ignition kinet-

cs is more relevant with compression ignition in engines. Thus our

rst objective is to extend previous ignition delay time study of

MF and MF to a wider range of equivalence ratios, pressures, and

uel concentrations. The new experimental data were compared

ith model predictions, and modifications were made to improve

odel performance in the low to intermediate temperature range.

n addition, the structural difference between DMF and MF makes

s to investigate the effect of side chain numbers. The reactivity of

MF and MF is compared in the low to intermediate temperature

ange. Kinetic analysis was further conducted to interpret the key

eaction pathways and dominant reactions. Furthermore, to eluci-

ate the potential engine applications of the fuel, comparison be-

ween DMF, PRF (primary reference fuel) components, and biofuels

n -butanol and DME (dimethyl ether)) was carried out based on

iterature data.

. Experimental specifications

.1. Setup and procedures

Ignition delay times of DMF and MF were measured using the

apid compression machine of Tsinghua University (TU-RCM). The

U-RCM consists of five components: the high pressure air tank,

he drive section, the hydraulic section, the driven section, and the

est section, see details in Refs. [34,35] . The length-variable text

ection permits convenient change of the compression ratio. The

est section is equipped with pressure transducer (KISLER6125C)

ogether with a charge amplifier (KISLER 5018A). A creviced piston

s employed to ensure a quasi-homogeneous temperature distribu-

ion after compression.

The mixture is prepared in a stainless steel tank according to

he partial pressure of each component. The purities of DMF and

F are 99% and 98%, respectively. A relative manometer (DPG400

mega) with an accuracy of 10 −5 MPa was used to measure the

artial pressure of the fuel. The vapor pressure of DMF is 7.07 kPa

t 295.3 K [36] and that of MF is 18.9 kPa at 293 K [37] . It is noted

hat the partial pressure of each fuel is assured to be less than

/3 of its saturated pressure so as to avoid condensation effect. Ar

nd N 2 were selected as dilution gas. At least one hour is waited to

chieve homogeneous mixing after charging the mixing tank. Since

MF is not very reactive at low temperatures, the lowest ignition

emperature was carefully examined for different equivalence ra-

ios and fuel concentrations at 16 bar. The lowest ignition tempera-

ure is achieved at the lowest compression ratio, and the Ar/N 2 ra-

io is then determined. Besides, the highest temperature of present

tudy is limited by the shortest ignition delay times can be validly

xtracted from the pressure trace. For a certain mixture compo-

ition, the end-of-compression gas temperature and pressure are

aried by varying the length of the test section and thus the com-

ression ratio. Experiments were conducted at least 3 min after fin-

shing charging the reaction chamber, letting the mixtures to reach

he equilibrium state. The corresponding unreactive mixture exper-

ments were conducted by replace O 2 with the same mole fraction

f N 2 to obtain volume-time profiles. The test mixture composition

s shown in Table 2.

.2. Definition of ignition delay times and test conditions

Figure 1 shows a typical pressure histories for the ignition

f DMF/O 2 /N 2 /Ar mixture (solid line) and the corresponding un-

eactive mixtures (dash line) where O 2 were replace with N 2 . It

s seen that the test section pressure increases sharply when the

iston approaches the top dead center and it reaches a maximum

P max = 29.81 bar) at the end of compression stroke. Then the pis-

on is locked at the top dad center, while the test section pressure

lightly decreases. This is caused by the heat loss to the test sec-

ion walls. Thus using the peak top dead center pressure P max and

max is not a reasonable way to demonstrate the test condition.

ere the effective pressure P eff and temperature T eff, as adopted

y Walton et al. [38] is used, as calculated in Eqs. (1) and ( 2 ). The

ixture ignite at τ ing = 166.3 ms, as manifested by the steep in-

rease in the pressure history. The effective pressure is obtained

rom the experimental pressure trace and is the integral average

218 N. Xu et al. / Combustion and Flame 168 (2016) 216–227

Fig. 1. Typical pressure trace obtained from TU-RCM.

Fig. 2. Typical pressure traces for pre-ignition heat release at 16 bar, 1% DMF, and

φ = 1.0.

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Fig. 3. Ignition delay times of DMF at 16 bar, equivalence ratio 0.5, and 0.5% fuel

concentration. Squares: ignition delay times obtained from XJTU shock tube [42] ;

circles: ignition delay times obtained from TU-RCM.

from the end-of-compression pressure to the lowest pressure in-

duced by heat loss.

P eff =

1

( t P min − t P max

)

∫ t P min

t P max

P dt (1)

The effective temperature is calculated using isentropic compres-

sion integral:

∫ T eff

T 0

γ

γ − 1

d ln T = ln

(P eff

P 0

)(2)

where T 0 and P 0 is respectively the initial temperature and pres-

sure before compression, and γ is the specific heat ratio of the test

mixture. The state condition is then calculated to be P eff = 27.01 bar

and T eff = 776.62 K. It is noted that the compression time is less

than 30 ms, and the definition of ignition delay time is the time

interval between the end-of-compression and the steepest rise of

pressure [38] .

We also note that pre-ignition heat release in rapid compres-

sion machines is of great importance since this would induce

pressure rise and heat release before ignition [39] . Mittal and

Sung [40] showed that pre-ignition heat release phenomena are

observed in the rapid compression machine pressure traces of

toluene and benzene. Tetralin exhibits pre-ignition heat release at

35 bar, which is more obvious with decreasing temperature [41] .

For alkylfurans whose molecular structures are similar to alkylben-

zenes, pre-ignition heat release was also observed by Uygun et al.

[33] in their optical shock tube measurement of MF ignition de-

lay times at 40 bar. In the present work, pre-ignition heat release

is observed for stoichiometric MF mixtures and DMF mixtures for

all the equivalence ratios studied. Typical pressure traces of reac-

tive and non-reactive tests for 1% stoichiometric mixtures at 16 bar

for two effective temperatures are shown in Fig. 2 . Small amount

of pressure rise could be identified before the sharp pressure rise

induced by the final ignition, if the pressure trace between the re-

active and unreactive cases are compared. However, this is differ-

ent from the abnormal runs caused by contaminated inner walls,

which is vulnerable especially at higher fuel loads. In those cases,

great amounts of pressure rise (as much as 50% the maximum

pressure due to ignition) would take place before the final ignition,

resulting in severe reduction in ignition delay times, as shown in

Fig. S2. Experimental runs suffered from pre-ignition heat release

are removed. The pre-ignition heat release observed here may be

related to the low reactivity of alkylfurans at lower temperature

range.

The simulation of the present ignition delay times were con-

ducted with the 0-dimensional reactor in CHEMKIN-II, and the ig-

nition delay time is defined as the time interval between the start

f calculation and the maximum d T /d t . The volume-time profiles

re included to account for the effect of heat loss. All species

bbreviations are according to the mechanism of Somers et al.

28,44] and tabulated in Supplemental material.

. Results and discussion

.1. Ignition delay times of DMF

The low to intermediated temperature ignition delay times of

MF were studied regarding various equivalence ratios, pressures,

nd fuel concentrations. The uncertainty of the present ignition

elay time measurements was estimated to be less than 11% and

hown in Fig. 3 . The main uncertainty in τ ign determination arises

rom the pressure measurement (which comes from the measure-

ent of initial pressure, the accuracies of pressure transducer, and

harge amplifier) and the uncertainties in temperature measure-

ent (which comes from measurement of pressure and the initial

emperature), while the mixture composition (which induces un-

ertainties in the determination of X O2 and X fuel ) only contributes

ittle. The total uncertainties in τ ign is calculated using the square

oot of the sum of the squares based on Eqs. (3) and (4) , see de-

ailed uncertainty analysis in Supplemental material.

The newly measured low to intermediate temperature (901–

143 K) ignition delay times of DMF at 16 bar and 0.5% fuel con-

entration were plotted together with the high temperature (1223–

617 K) ignition delay times data measured using the shock tube of

i’an Jiaotong University [42] in Fig. 3 . The detailed introduction of

he shock tube facility used in this work can be found in Ref. [43] ,

ith an uncertainty of ±21% for ignition delay times. As shown in

N. Xu et al. / Combustion and Flame 168 (2016) 216–227 219

Fig. 4. Experimental and correlated ignition delay times of stoichiometric DMF at

16 and 30 bar under 1% fuel concentration. Symbols: experimental ignition delay

times of DMF with 11% error bar; solid line: correlated values.

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Fig. 5. Effect of equivalence ratio on the ignition delay times of DMF for 1% fuel

concentration with 11% error bar: (a) P = 16 bar and (b) P = 30 bar.

Fig. 6. Effect of fuel concentration on the ignition delay times of stoichiometric

DMF at 16 bar. Symbols: experimental ignition delay times of DMF with 11% error

bar; solid line: the modified mechanism of Somers et al. [28,44] ; dash line: the

mechanism of Somers et al. [28,44] ; dot line: the mechanism Liu et al. [18] .

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ig. 3 , the ignition delay times measured by present work show

onsistence with those measured by XJTU shock tube. It can be

een from the RCM measured data that when the temperature is

ower than 900 K, the ignition delay times versus the inverse tem-

erature has a decreased slope, indicating that the overall activa-

ion energy of ignition is slightly lower. This phenomenon could

e observed under other experimental conditions for DMF as well.

o NTC (negative temperature coefficient) behavior was observed

n the present experimental range, the same as that observed in

SR (jet stirred reactor) species profiles [28] .

To correlate present ignition delay times with experimental pa-

ameters and reflect the alternation in activation energy with de-

reasing temperature, two correlations ( Eqs. (3) and ( 4 )) are pro-

ided for ignition temperatures higher and lower than 900 K, re-

pectively.

> 900 K

ign = 1 . 66 × 1 0

−5 P −1 . 396 X O 2 −1 . 454 X fuel

−0 . 179 exp

(25 . 307

RT

)(3)

< 900 K

ign = 4 . 31 × 1 0

−5 P −1 . 429 X O 2 −0 . 997 X fuel

−0 . 361 exp

(24 . 175

RT

)(4)

here τ ign stands for the ignition delay time in millisecond, T is

he temperature in Kelvin, P is the pressure in bar, φ is the equiv-

lence ratio, and X fuel is the fuel concentration. Figure 4 shows

he measured and correlated ignition delay times of DMF at 16

nd 30 bar, and the ignition delay times increase with the decreas-

ng temperature. Though the ignition delay times at low tempera-

ures are slightly over-predicted, the correlation can give general

ood agreement against experimental results both quantitatively

nd qualitatively.

.1.1. The effect of equivalence ratio

The effect of equivalence ratio on the ignition delay times of

MF was studied at pressures of 16 and 30 bar. Logarithmic ig-

ition delay times of DMF at equivalence ratios of 0.5, 0.75, 1.0,

nd 2.0 and at 16 bar versus inverse temperature were depicted in

ig. 5 (a). It can be seen from the figure that the ignition delay

imes increase with the increasing equivalence ratio, and the de-

endence of the equivalence ratio is weakened when the tempera-

ure is decreased, especially at higher pressure shown in Fig. 5 (b).

he dependence of low to intermediate temperature ignition delay

imes of DMF on equivalence ratio is the same as that observed

n the high temperature ignition delay times study of Sirjean et al.

26] .

.1.2. The effect of fuel concentration

The influence of fuel concentration on the ignition delay times

f DMF was also investigated. The alkylfuran kinetic mechanisms

f Somers et al. [28,44] and Liu et al. [18] were employed to

imulate the ignition process. The mechanism of Somers et al.

28,44] (dash line) has been previously validated against experi-

ental ignition delay times, laminar flame speeds, pyrolysis, and

SR species profiles. The mechanism of Liu et al. [18] has been val-

dated against low pressure laminar premixed flame structures of

MF, MF, and furan. It is noted that when the mechanism of Liu

t al. [18] is applied (dot line), the low pressure rate constants

ere replaced with the corresponding high pressure limit values

btained from Sirjean et al. [26] . These two mechanisms have not

een sufficiently validated against low to intermediate temperature

gnition data of DMF. Figure 6 shows the measured and calculated

220 N. Xu et al. / Combustion and Flame 168 (2016) 216–227

Fig. 7. Experimental and computed ignition delay times of DMF for 1% fuel concen-

tration: (a) φ = 0.5; (b) φ = 0.75; (c) φ = 1.0 and (d) φ = 2.0. Symbols: experimental

ignition delay times of DMF with 11% error bar; solid line: the modified mechanism

of Somers et al. [28,44] ; dash line: the mechanism of Somers et al. [28,44] ; dot line:

the mechanism of Liu et al. [18] .

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stoichiometric ignition delay times of DMF with fuel concentra-

tions of 0.5%, 1%, and 2.0% at 16 bar. The decrease of fuel concen-

tration prolongs the ignition delay times of DMF, as indicated both

by experimental data and simulation. The mechanisms of Somers

et al. [28,44] and Liu et al. [18] could correctly capture the trend,

while model predictions deviate from experimental data to some

extent. The mechanism predictions of Somers et al. [28,44] shows

notable curvature, and the slope of Liu et al.’s [18] prediction dif-

fers from those measured.

3.1.3. The effect of pressure

The pressure dependence of DMF ignition delay times was stud-

ied at different equivalence ratios. Figure 7 (a) shows experimental

and simulated ignition delay times of DMF at equivalence ratio of

0.5 and pressures of 16 and 30 bar. It can be seen that the increase

in pressure induces increased reactivity. Figure 7 (b)–(d) shows the

same trend under equivalence ratios of 0.75, 1.0, and 2.0. The

mechanism of Liu et al. [18] consistently shows deviated slopes

compared with experimental data, and this deviation is more sig-

nificant at lower temperatures. Albeit the deviation from experi-

mental data, the mechanism of Somers et al. [28,44] shows better

agreement. However, the mechanism of Somers et al. [28,44] pro-

vides exaggerated curvatures, especially at equivalence ratios of

0.75 and 1.0. We have attempted to improve the model perfor-

mance based on the mechanism of Somers et al. [28,44] , modifi-

cations have been made with respect to the primary fuel oxidation

pathways, as the follows.

3.2. Mechanism modification

The present work first look at H-abstraction reactions of DMF

and MF, which is of great importance both at low and high

temperatures. The rate constants of seven H-abstraction reac-

tions have been updated according to the PhD thesis of Somers

[45] . It should be noted that after the rate constant of R107

(DMF25 + O 2 = DMF252J + HO 2 ) has been updated, the curvatures

in predictions have been greatly alleviated, inducing that the

mechanism could qualitatively capture the experimental data. The

abbreviations for all the species were presented as the Supplemen-

tal material.

Besides, in the original mechanism, H-abstraction reactions of

MF25CHO (5-methylfurfural) with CH 3 , HO 2 , and CH 3 O 2 were

obtained in analogy with reactions of toluene, which might be

higher by one order of magnitude [46] . Decreasing the A-factors

of MF25CHO + CH 3 = MF25CJO + CH 4 , MF25CHO + HO 2 = MF25CJO +H 2 O 2 , and MF25CHO + CH 3 O 2 = MF25CJO + CH 3 O 2 H by a factor of 5

prolongs the ignition delay times.

The reaction of DMF252J radical with HO 2 radical generating

DMF252OJ and OH radical enhances the low to intermediate tem-

perature reactivity and is of great importance in the ignition de-

lay time prediction. The A-factor of R126: DMF252J + HO 2 =DMF252OJ + OH has been decreased by a factor of 1.5, within the

uncertainty of this reaction, which is 3–4 [28] . The details of mod-

ification are presented in Table 3.

The performance of the modified mechanism is depicted as

solid lines in Figs. 6 and 7 . As shown in Fig. 6 , after the above men-

tioned modifications have been incorporated, the modified mecha-

nism of Somers et al. [28,44] agree better with experimental data,

especially at higher fuel loads of 1% and 2%. Compared to Somers’s

mechanism [28,44] and Liu’s mechanism [18] , the modified mech-

anism shows better agreement with the experimental data for

equivalence ratios of 0.75 ( Fig. 7 (b)) and 1.0 ( Fig. 7 (c)). However,

for the equivalence ratios of 0.5 Fig. 7 (a)) and 2.0 ( Fig. 7 (d)), the

modified mechanism could not provide good agreement for lean

mixtures at lower temperature and 30 bar and rich mixtures at

higher temperatures and 16 bar. We have tried to artificially change

he rate constants for the reactions DMF25 + O 2 = DMF252J + HO 2 ,

MF252J + HO 2 = DMF252OJ + OH, etc., which are important for

he present ignition delay time predictions and if changed ar-

ificially, we can get better agreement with our experimentally

N. Xu et al. / Combustion and Flame 168 (2016) 216–227 221

Table 3

Updated reaction rate parameters employed to the alkylfuran mechanism of Somers et al. [28,44] . The unit of Ea is

J/mol.

Reaction A n Ea A new n new Ea new

R107 DMF25 + O 2 = DMF252J + HO 2 a 1.25e13 0.0 35,251 5.60e03 3.3 35,229

R108 DMF25 + O = DMF252J + OH

a 1.26e12 0.0 30 0 0 1.26e12 0.0 0

R111 DMF25 + C 3 H 3 = DMF252J + C 3 H 4 -P a 3.20e12 0.0 15,100 8.00e11 0.0 9993

R112 DMF25 + C 3 H 3 = DMF252J + C 3 H 4 -A a 3.20e12 0.0 15,100 8.00e11 0.0 9993

R114 DMF25 + C 5 H 5 = DMF252J + C 5 H 6 a 3.20e12 0.0 11,0 0 0 3.20e12 0.0 15,091

R434 MF2 + O = MF22J + OH

a 6.30e11 0.0 30 0 0 6.30e11 0.0 0

R442 MF2 + C 5 H 5 = MF22J + C 5 H 6 a 1.60e12 0.0 11,100 1.60e12 0.0 15,100

R126 DMF252J + HO 2 = DMF252OJ + OH 5.00e12 0.0 0 3.30e12 0.0 0

a Doctoral thesis of Somers [45] .

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easured ignition delay times. However, the performance of pre-

icting the species profiles of pyrolysis, JSR, and high temper-

ture ignition delay times are getting worse. Thus we propose

hat further refinements on the rate constants of such reactions

s DMF25 + O 2 = DMF252J + HO 2 , DMF252J + HO 2 = DMF252OJ + OH,

nd DMF252J + CH 3 O 2 = DMF252OJ + CH 3 O, especially their pres-

ure dependence should be conducted using quantum chemistry

ethods to provide better agreement. The performance of the

odified mechanism on predicting the JSR species profiles [28] is

epicted in Fig. 8 . It is seen that for most species, the modi-

ed mechanism shows better predictions. Further validation of the

odified mechanism against pyrolysis profiles [28] and high tem-

erature ignition delay times [26,28] is provided in the Supple-

ental material.

.3. Comparison between DMF and MF

The low to intermediate temperature ignition delay times of

F was also measured at the equivalence ratio of 1.0 and at pres-

ures of 16 and 30 bar for 1% fuel concentration. Figure 9 (a) shows

he comparison between the stoichiometric ignition delay times

f DMF and MF at 16 bar. It is seen that at low to intermedi-

te temperature conditions, the logarithmic ignition delay times of

F shows a quasi-linear dependence on the inverse temperature.

hile for DMF, as discussed in the previous section, the ignition

elay time shows a slight bending down as temperature decreases.

n addition, the overall reactivity of MF is slightly more sensitive to

emperature than that of the DMF. The modification to the mech-

nism of Somers et al. [28,44] show little influence on the agree-

ent with the measured ignition delay times of MF. In addition,

oth experimental data and the simulations show that there is a

rossover temperature, above which the ignition delay times of MF

s lower than that of DMF and vice versa. Similar phenomenon

s also observed at a higher pressure of 30 bar, as shown in

ig. 9 (b). Interestingly, we have previously reported [42] that at

igh temperatures ( > 1150 K), the relative reactivity of DMF and

F depends on temperature also, and another crossover temper-

ture was observed for DMF and MF at the fuel concentration of

.5%, which moves to higher temperatures with increasing pres-

ure ( Fig. 11 in Ref. [42] ). In other words, one may expect that

here are two intersections between the ignition delay times of

MF and MF through the whole temperature range. Calculated ig-

ition delay times using the modified mechanism and the origi-

al mechanism of Somers et al. [28,44] at 16 bar for the same fuel

oncentration is shown in Fig. 9 (c) and the two intersections ( T c1

nd T c2 ) at high and low temperatures are observed, respectively.

e note that experimentally, the high temperature ignition delay

imes using the shock tube for higher fuel concentration (1%) at

6 bar could not be collected because the ignition delay times at

his condition are is so short ( < 50 μs)) that the method in Ref.

42] is no more valid for accurate ignition delay time determina-

ion. Thus we do not have direct high temperature experimental

ata at the fuel concentration of 1% at 16 bar for comparison. The

inetic interpretations for the relative reactivity of MF and DMF

t temperatures close to T c 1 have been presented in detail in Ref.

42] . Here we looked into the ignition kinetics of MF and DMF at

ower temperatures (close to T c2 ).

Figure 10 (a) depicts the major reaction pathways of DMF under

6 bar and 20% fuel consumption at 880 K and 1030 K using the

odified mechanism. At 880 K, H-abstraction reactions from the

ethyl side consumes 36.6% DMF (most by CH 3 and OH radicals),

ollowed by OH-addition reactions to yield DMF252OH3J (27.2%)

nd H-additions to form H4E2O3J (26.2%), different from high

emperature conditions [26,28] in which DMF is mainly consumed

y H-abstractions, H-additions, and ipso-addition reactions. The

rimary fuel radicals DMF252J mainly go through: DMF252J →MF252OJ → MF25CHO → MF25CJO → MF25J → P3E25O5J → CH 2

HCH 2 CO and subsequent oxidation reactions. DMF252J radicals

ould add with CH 3 radicals generating M2E5F as well. Moreover,

he typical low temperature chain branching routine does not

ominate the ignition kinetics. The DMF252J radical goes through

xygen addition reaction, but the subsequent isomerization reac-

ions leading to low temperature chain branching or HO 2 elimina-

ion reactions were not important. Simmie et al. [47] pointed out

hat the reactions of DMF252J + O 2 would lead to the formation

f hydroperoxide or aldehyde and OH radical. However, the back

issociation reactions and reactions yielding CH 2 O (formaldehyde)

ave higher rate constants. The reaction DMF252J with O 2 and

ubsequent reactions were thoroughly examined by Simmie et al.

47] , and the pathways is depicted in Fig. 11 . The activation energy

f the addition reaction of 5-methyl-2-furanylmethyl radical with

xygen yielding peroxide radicals (DMF252J + O 2 =MF25CH2OO) is

mall (3.7 kcal/mol), which renders the reaction feasible at lower

emperatures. However, the transition state in the subsequent

somerization from peroxide radical to hydroperoxide radical

nvolves bicyclic structures with high strain energy. In addition,

here is less available H atoms (due to double bonds on the

ing) that can participate in this sequence. High C-H bond energy

∼120 kcal/mol) on the ring maybe also result in a high activation

nergy of this process. The activation energy of this sequence is

o high (41.8 kcal/mol) that the internal H-transfer isomerization

eaction of MF25CH2OO seldom takes place at low temperature.

his is consistent with the statement of Curran et al. [48] that

eactions with higher barriers (27–40 kcal/mol) are not favored at

ow temperature. Subsequent reactions leading to low temperature

hain branching are thus not favored.

This explains the phenomenon observed in Fig. 5 that ignition

elay times of DMF increase with increasing equivalence ratio. In

he low temperature range, if the low temperature routine dom-

nates the ignition, the chain branching reactions are directly re-

ated with fuel molecules and the subsequent chain branching re-

ctions after fuel radical addition with oxygen are crucial, induc-

ng longer ignition delay times with decreasing equivalence ratios,

hich is not the present case.

222 N. Xu et al. / Combustion and Flame 168 (2016) 216–227

Fig. 8. Experimental and simulated species profiles of JSR [28] at φ = 1.0, 10 atm

and τ = 0.7 s: (a) DMF, CH 4 , and H 2 ; (b) H 2 O, CO, CO 2 , and O 2 ; (c) CH 2 O, C 2 H 6 ,

C 2 H 4 , and C 2 H 2 and (d) methyl vinyl ketone, 5-methyl-2-ethylfuran, and 5-methyl-

2-fomylfuran. Solid line: the modified mechanism of Somers et al. [28,44] ; dash

line: the mechanism of Somers et al. [28,44] .

Fig. 9. Comparison among ignition delay times of stoichiometric DMF and MF

under 1% fuel concentration. (a) Experimental results and simulations at 16 bar;

(b) experimental results and simulations at 30 bar and (c) simulation in the whole

temperature range. Symbols: experimental ignition delay times; solid line: the mod-

ified mechanism of Somers et al. [28,44] ; dash line: the mechanism of Somers et al.

[28,44] .

t

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a

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s

Instead, the DMF252J radical would go disproportionation reac-

ions with CH 3 O 2 or HO 2 radicals to produce DMF252OJ radicals.

hus reaction R126: DMF252J + HO 2 = DMF252OJ + OH and R127:

MF252J + CH 3 O 2 = DMF252OJ + CH 3 O are of great importance

n the low temperature oxidation of DMF. R126 and R127 con-

umes 39.6% and 44.8% DMF252J radical. From the sensitivity anal-

sis conducted at the same conditions, as shown in Fig. 12 (a),

hese two reactions have the largest negative sensitivity coeffi-

ients. Compared with high temperature conditions where large

mount of stable species such as phenol, 1, 3-cyclopentadiene, and

ther aromatics are formed, the open of furan ring would yield

mall hydrocarbons at the low temperature conditions.

N. Xu et al. / Combustion and Flame 168 (2016) 216–227 223

Fig. 10. Reaction pathway analysis for stoichiometric 1% fuel mixture at 16 bar, and 20% fuel consumption using the modified alkylfuran mechanism of Somers et al. [28,44] .

Normal font: 880 K; bold font: 1030 K. (a) DMF and (b) MF.

224 N. Xu et al. / Combustion and Flame 168 (2016) 216–227

Fig. 11. Primary reaction pathways of MF25CH2OO, adapted from Simmie et al.

[47] . The energy barrier (kcal/mol) of each step was presented.

Fig. 12. Sensitivity analysis for stoichiometric 1% fuel mixture at 16 bar, 20% fuel

consumption, 880 and 1030 K. (a) DMF and (b) MF. Mechanism used: the modified

alkylfuran mechanism of Somers et al. [28,44] .

e

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c

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Figure 10 (b) shows the reaction pathways of MF at the same

conditions. As to MF at 880 K, H-abstraction reactions account for

28.8% fuel consumption which yields MF22J radical, followed by

OH addition at the C2 or C5 position (23.5% and 23.7%, respec-

tively). Because of the similar molecular structures of DMF and MF,

the MF22J radical goes in an analogy pathway with DMF252J rad-

ical: MF22J → MF22OJ → F2CHO → F2CJO → F2J and subsequent de-

composition reactions or add with CH 3 radial yielding E2F. For

both fuels, when it goes to a higher temperature of 1030 K, the

proportions for OH-addition reactions decrease, while those of H-

abstractions and ipso-addition reactions increase, which is much

similar to high temperature reaction pathways [42] .

Since the structures of DMF and MF are similar to toluene

(ring-structured, double bond on the ring, and methyl side chains),

their reactions schemes might be analogous to some extent. Silva

t al. [49] and Murakami et al. [50] have studied the reactions of

ethylphenyl radical and benzyl radical with oxygen, respectively.

either of their investigations showed the dominance of isomer-

zation of peroxy species to hydro peroxy species and subsequent

hain branching reaction, just like methylfurans. It is worth not-

ng that before the ring opens, major pathways taking place at

he methyl side chains are exactly the same to those of toluene

40,51,52] , suggesting that the reaction schemes of alkylbenzenes

an be referenced when developing mechanisms of larger alkyl-

urans. Experimentally, Roubaud et al. [53] pointed out that low

lkylbenzenes are reactive only when they possess adjacent/long

ide chains, which might also be true for larger alkylfurans.

Sensitivity analysis is further performed to identify the control-

ing reactions in the autoignition process. Figure 12 illustrates the

ensitivity analysis conducted at 880 and 1030 K, 16 bar, and 1%

uel concentration for DMF and MF, respectively. The sensitivity co-

fficient is calculated through individually increasing and decreas-

ng the pre-exponent constant by a factor of 2:

ensitivity coefficient = ( τ2 k − τ0 . 5 k ) / τk (5)

A negative sensitivity coefficient denotes that the reaction

lays a promoting role in ignition and vice versa. For DMF, as

hown in Fig. 12 (a), unlike at high temperature conditions, R1357:

+ O 2 = O + OH possessed only a moderate sensitivity among these

ost sensitive 22 reactions. R126: DMF252J + HO 2 = DMF252OJ +H shows the highest negative sensitivity, denoting that the re-

ction acts to promote ignition strongly. When the temperature is

nhanced from 880 K to 1030 K, sensitivities of DMF252OJ related

eactions begin to decrease, while those of DMF252J and H 2 O 2 re-

ated reactions show an increase: R104, R106, and R109 begin to

e sensitive. It could be observed from Fig. 12 (b) that the above-

entioned features are also true for MF. Besides, OH-addition

eaction: R588: MF2 + OH = MF22OH3J is sensitive for ignition

ither.

The above reaction flux and sensitivity analysis at the temper-

tures close to T c2 have indicated that due to the similarity of the

olecular structures of DMF and MF, their overall ignition kinet-

cs are very similar. Compared with MF, DMF has one more lat-

ral methyl side chain, while the two lateral methyl groups are

solated by two ring carbon atoms. Roubaud et al. [53] concluded

hat at low to intermediate temperatures, the overall reactivity

f alkylbenzenes is determined by the proximity or length rather

han the number of aliphatic carbon atoms. Considering the sim-

larity of alkylfurans and alkylbenzenes, it is reasonable that only

mall difference exists between the reactivity of DMF and MF since

heir structures only differs in the number of isolated methyl side

hains. Moreover, the slight curve like ignition delay times of DMF

ight suggest that alkylfurans with adjacent or longer alkyl side

hains would exhibit ignition behaviors which is similar to alkanes

r alkenes.

.4. Comparison between DMF and primary reference fuel

As a promising candidate for fossil fuels, the fundamental com-

ustion characteristics of DMF have been extensively investigated.

rior to its practical application, the comparison between DMF and

RF components ( iso -octane, n -heptane, and toluene) and other

iofuels ( n -butanol and DME (dimethyl ether)) at low to interme-

iate temperature range is of importance, indicating the potential

sage of DMF in internal combustion engines. Previously, Eldeeb

t al. [30] declare that the high temperature ignition delay times

f DMF are longer than those of iso -octane, while the compari-

on under low to intermediate temperature range is rare. Compari-

on among ignition delay times of DMF, iso -octane, toluene, and n -

eptane is conducted under constant fuel concentration conditions,

espectively. Stoichiometric ignition delay times of iso-octane/air

N. Xu et al. / Combustion and Flame 168 (2016) 216–227 225

Fig. 13. Comparison between ignition delay times of DMF and PRF. (a) Iso -octane

and DMF (present work correlated). Solid squares: present DMF data; half hollow

circles: iso -octane data of Davidson et al. [54] ; half hollow triangles: iso -octane data

of Mansfield et al. [55] ; half hollow squares: iso -octane data of Di et al. [34] . (b)

Toluene (correlated using the correlation of Mittal et al. [40] ) and DMF (present

work) at equivalence ratios of 0.5, 0.75, and 1.0. Solid symbols: DMF; half hollow

symbols: toluene and (c) N-heptane (Silke et al. [56] ) and DMF (correlated from

present work). Solid symbols: DMF; half hollow symbols: n -heptane.

m

M

d

c

i

d

s

Fig. 14. Comparison between ignition delay times of DMF and biofuels. (a) n -

butanol and DMF (correlated from present work). Squares: n -butanol data of Weber

et al. [57] ; circles: DMF; (b) DME and DMF (correlated from present work). Squares:

DME data from Mittal et al. [63] ; circles: DMF.

r

t

s

e

f

i

b

F

l

a

e

a

n

o

l

p

r

t

s

w

t

s

t

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p

t

D

ixtures measured by Davidson et al. [54] , Di et al. [34] , and

ansfield et al. [55] were compared with corresponding ignition

ata of DMF correlated using Eqs. (3) and (4) . From Fig. 13 (a), it

an be found that DMF shows longer ignition delay times, which

s 2–3 times the values of iso -octane. For toluene, the present DMF

ata were compared with correlated values of Mittal et al. [40] , as

hown in Fig. 13 (b). For all the three equivalence ratios, toluene is

elatively unreactive compared with DMF, consistent with the fact

hat low temperature isomerization from ROO to QOOH is quite

low for toluene [49,50] . As to n -heptane in Fig. 13 (c), the refer-

nce fuel for diesel engine, more than an order of magnitude dif-

erence exists between its ignition data and those of DMF, reveal-

ng their significant difference in reactivity.

With respect to biofuels, comparison between DMF and n -

utanol or DME is shown in Fig. 14 (a) and (b), respectively. From

ig. 14 (a) it can be observed that the reactivity of DMF is simi-

ar with that of n -butanol. The ignition delay times of DMF are

bout 10 times those of DME. If we look at the RONs of the fu-

ls studied (see Table 4 ), the RONs of iso-octane, n -butanol, DMF,

nd toluene are close to or higher than 100, while the RONs of

-heptane and DME are no more than 0. The controlling kinetics

f DMF and toluene at low to intermediate temperatures are simi-

ar, going through a series of reactions including H-abstraction, dis-

roportation, H radical release, H-abstraction, CO elimination, and

ing open reactions. For n -butanol [57,58] at relevant experimen-

al conditions, the fuel would go through H-abstractions at the αite, and most of the resulted radicals would undergo H-abstraction

ith O 2 to give HO 2 radicals rather than typical low tempera-

ure chain branchings. H-abstractions by H and OH radicals and

ubsequent β-scissions dominate the ignition of iso-octane at in-

ermediate temperatures [59] . Inversely, both n -heptane and DME

60] would experience typical low temperature oxidation schemes

roposed by Curran et al. [48] . The undominance of typical low

emperature oxidation reactions separate DMF from n -heptane and

ME which are CI engine candidates. From the above analysis, DMF

226 N. Xu et al. / Combustion and Flame 168 (2016) 216–227

Table 4

RONs of the fuels studied and compared.

Fuel DMF MF Toluene iso -octane n -heptane n -butanol DME

RON 119 a 131 a 120 b 100 c 0 d 96 c / e

a Collected from Leshkov et al. [1] . b Collected from Shen et al. [64] . c Collected from Serras-Pereira et al. [65] . d Collected from Silke et al. [56] . e Collected from Ji et al. [66] .

A

d

N

f

e

P

n

S

f

0

R

could serve as an SI engine candidate or an additive in CI engines

for knock resistance, in accordance with the conclusion of Sudholt

et al. [61] that furanic fuels are suitable for SI engines.

Furthermore, Tran et al. [62] adopted sampling techniques to

study the flame structure of low pressure rich laminar premixed

flames of DMF and MF. Their results indicate that the tendency to

form soot precursors of DMF is lower than that of toluene although

they possess similar octane numbers, and that of MF is the lowest.

The study indicate that alkylfurans may be better octane improver

in gasoline compared with aromatics.

4. Concluding remarks

Low to intermediate temperature auto-ignition behavior of DMF

has been investigated at pressures of 16 and 30 bar and tempera-

ture range between 737–1143 K using TU-RCM. Effects of equiva-

lence ratio and fuel concentration have been investigated. Corre-

sponding ignition delay times of 1% MF mixture is measured at

16 and 30 bar for comparing the combustion chemistry of these

two fuels. The ignition delay times of DMF increase with increas-

ing equivalence ratio, suggesting that the typical low temperature

chain branching reactions are not dominant due to the high activa-

tion energy involved in the isomerization of peroxy radicals to hy-

droperoxy radicals. In the low to intermediate temperature range,

the reactivity of DMF is close to that of MF. A cross over tempera-

ture is observed, below which the reactivity of DMF is higher and

vice versa. We have previously observed another crossover temper-

ature between the ignition delay times of these two fuels at high

temperature range. It is indicated that two crossover temperatures

existed in the whole temperature range, since the dependence of

ignition delay times of DMF on temperature decreases with de-

creasing of temperature, while that dependence of MF maintains

unchanged.

The present data are of importance to validate the existed alkyl-

furan mechanisms against low to intermediate temperature igni-

tion delay times. Deviations are observed between experimental

data and predictions of two mechanisms. Based on the mecha-

nism of Somers et al. [28,44] , 10 H-abstractions are updated and

the rate constant of R126: DMF252J + HO 2 =DMF252OJ + OH is de-

creased by a factor of 1.5. When modifications are incorporated,

the alkylfuran mechanism of Somers et al. [28,44] could capture

experimental ignition delay times both qualitatively and quanti-

tatively. Using the modified alkylfuran mechanism, reaction path-

way analysis reveals that DMF and MF are consumed mainly in

a quite similar way: H-abstraction, disproportionation, H radical-

release, H-abstraction, and subsequent decomposition reactions, in

analogy with toluene. Comparison among DMF, PRF components,

and biofuels is made under constant fuel concentrations. Under

conditions examined, the reactivity of DMF is higher than toluene,

close to iso -octane and n -butanol, and much lower compared with

n -heptane and DME, indicating that DMF is suitable for SI engines

or add as knock resistance for CI engines. Further ignition data of

DMF concerning high pressures and low temperatures are needed

to validate and optimize kinetic mechanisms more elaborately.

cknowledgments

This work is supported by the National Natural Science Foun-

ation of China ( 91541107 , 51206131 , and 91441203 ), and the

ational Basic Research Program ( 2013CB228406 ). The support

rom the State Key Laboratory of Automotive Safety and En-

rgy ( KF14102 ) was also acknowledged. The authors thank Kieran

atrick Somers for the helpful discussion about the kinetic mecha-

ism.

upplementary materials

Supplementary material associated with this article can be

ound, in the online version, at doi:10.1016/j.combustflame.2016.03.

16 .

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