This document describes an experimental study that measured ignition delay times for fuel-air mixtures behind reflected shock waves under conditions relevant to homogeneous charge compression ignition (HCCI) engines. Ignition delay times were measured for n-heptane, gasoline, and a gasoline surrogate mixture (iso-octane/toluene/n-heptane) over a range of pressures, temperatures, equivalence ratios, and exhaust gas recirculation levels using pressure history and chemiluminescence diagnostics in shock tubes. The results provide benchmark data for validating detailed chemical kinetic models under HCCI combustion conditions.

1. Combustion and Flame 139 (2004) 300–311
www.elsevier.com/locate/jnlabr/cnf
Shock tube determination of ignition delay times
in full-blend and surrogate fuel mixtures
B.M. Gauthier, D.F. Davidson ∗
, R.K. Hanson
Mechanical Engineering Department, Building 520, Duena Street, Stanford University, Stanford, CA 94305-3032, USA
Received 30 January 2004; received in revised form 26 July 2004; accepted 26 August 2004
Available online 27 October 2004
Abstract
Autoignition characteristics of n-heptane/air, gasoline/air, and ternary surrogate/air mixtures were studied be-
hind reflected shock waves in a high-pressure, low-temperature regime similar to that found in homogeneous
charge compression ignition (HCCI) engine cycles. The range of experiments covered combustion of fuel in air
for lean, stoichiometric, and rich mixtures (Φ = 0.5, 1.0, 2.0), two pressure ranges (15–25 and 45–60 atm), tem-
peratures from 850 to 1280 K, and exhaust gas recirculation (EGR) loadings of (0, 20, and 30%). The ignition
delay time measurements in n-heptane are in good agreement with the shock tube study of Fieweger et al. (Proc.
Combust. Inst. 25 (1994) 1579–1585) and support the observation of a pronounced, low-temperature, NTC region.
Strong agreement was seen between ignition delay time measurements for RD387 gasoline and surrogate (63%
iso-octane/20% toluene/17% n-heptane by liquid volume) over the full range of experimental conditions studied.
Ignition delay time measurements under fuel-lean (Φ = 0.5) mixture conditions were longer than with Φ = 1.0
mixtures at both the low- (15–25 atm) and high- (45–60 atm) pressure conditions. Ignition delay times in fuel-rich
(Φ = 2.0) mixtures for both gasoline and surrogate were indistinguishable in the low-pressure (15–25 atm) range,
but were clearly shorter at high-pressures (45–60 atm). EGR loading affected the ignition delay times similarly
for both gasoline and surrogate, with clear trends indicating an increase in ignition delay time with increased EGR
loading. This data set should provide benchmark targets for detailed mechanism validation and refinement under
HCCI conditions.
 2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
Keywords: Ignition times; n-Heptane; Gasoline; Gasoline surrogate; Shock tube
1. Introduction
Homogeneous charge compression ignition (HC-
CI) offers the potential of simultaneously increasing
fuel efficiency in internal engines as well as reduc-
ing harmful emissions such as NOx and particulate
matter. Many HCCI combustion strategies have been
* Corresponding author. Fax: +1-650-723-1748.
E-mail address: dfd@stanford.edu (D.F. Davidson).
studied by various investigators and included in these
studies are efforts to develop and improve detailed
chemical mechanisms suitable for modeling HCCI
combustion. To validate and refine these computer
models, detailed kinetic measurements, including ig-
nition times and species concentration time histories,
are needed at higher pressures (of order 50 atm) and
lower temperatures (of order 900 K).
Although HCCI combustion can be tailored to op-
erate on nearly any type of hydrocarbon or alcohol
0010-2180/$ – see front matter  2004 The Combustion Institute. Published by Elsevier Inc. All rights reserved.
doi:10.1016/j.combustflame.2004.08.015

2. B.M. Gauthier et al. / Combustion and Flame 139 (2004) 300–311 301
fuel, gasoline is particularly attractive because of its
well-established infrastructure. Compared to single-
component fuels, however, a distillate fuel such as
gasoline offers the additional complexities of hav-
ing a near-continuous spectrum of hydrocarbon con-
stituents, as well as a composition that can vary from
sample to sample. In these cases it is beneficial to em-
ploy a surrogate fuel with a finite number of compo-
nents and a standard composition to analyze detailed
chemical models. It should be noted, however, that
gasoline surrogates optimized or simplified for chem-
ical modeling of ignition delay times are not usually
the optimal surrogate mixtures to simulate other per-
formance observables in real engines.
Kinetic targets for model validation and refine-
ment are needed for many fuels: gasolines, surro-
gate blends, and individual fuel components. We
have found no shock tube data on gas-phase gaso-
line ignition time measurements in the HCCI regime.
However there are many studies of surrogate fuels
and fuel components in the combustion literature.
HCCI ignition has been studied in a rapid com-
pression machine study by Tanaka et al. [1,2] for
the ignition of various pure hydrocarbon fuels in-
cluding n-heptane, iso-octane, cyclo-alkanes, olefins,
and cyclo-olefins, and primary reference fuel blends
of n-heptane and iso-octane. Shock tube studies at
high pressures have been performed by Ciezki and
Adomeit on n-heptane/air [3] and by Fieweger et al.
[4,5] on n-heptane, iso-octane, and other fuel com-
ponents, as well as various blends of iso-octane and
n-heptane. A similar study of diesel-relevant fuels
(α-methylnaphthalene, n-decane, and dimethylether)
was performed by Pfahl et al. [6]. However, all these
existing high-pressure shock tube data come from one
laboratory group only (Aachen) and has not been ver-
ified.
In the past several years in our laboratory, we have
been advancing a program to improve the accuracy,
reliability, and understanding of ignition time stud-
ies in shock tubes, particularly in the area of ignition
time definition, mixture formation, available test time,
diagnostics, and modeling [7]. Using this improved
methodology, we have measured ignition delay times
for n-heptane, gasoline, and surrogate blends of iso-
octane/toluene/n-heptane for a variety of conditions
suitable for comparison with HCCI kinetic models
and present the results here.
2. Experimental method
Shock tube ignition delay time measurements
were performed in the High Temperature Gasdynam-
ics Laboratory at Stanford University. The facilities
and procedures used for this study are outlined in
the following sections: Section 2.1, experimental fa-
cilities; Section 2.2, available test time; Section 2.3,
measuring ignition delay times; Section 2.4, assump-
tion of vibrational relaxation; Section 2.5, mixture
preparation for liquid fuels; and Section 2.6, thermo-
dynamic surrogate for gasoline.
2.1. Experimental facilities
All experiments were performed in the High Pres-
sure Shock Tube (HPST) facility with the exception
of the low-pressure n-heptane ignition delay times.
The procedure for the low-pressure n-heptane exper-
iments is essentially the same as outlined below, al-
though the tests were performed in a larger diameter
(15.2 cm) shock tube.
Ignition delay time experiments were performed
in a 5.0-cm-diameter, helium driven, turbo-molecular
pumped, high-pressure shock tube facility at Stan-
ford University. Six piezo-electric pressure transduc-
ers (PZT) (PCB Model 113A26) were located axially
along the driven section to trigger five, fast-response
time-interval counters for the measurement of the av-
erage incident shock speed over five different inter-
vals. The incident shock speed at the endwall was de-
termined by a linear extrapolation of the axial velocity
profile. Typical attenuation rates for this experiment
ranged from 1 to 3% /m. Reflected shock conditions
were determined using the one-dimensional normal
shock equations and the Sandia thermodynamic data-
base by Kee et al. [8], including additional species
information as recommended by Burcat [9].
Prior to experimentation, a series of pure argon
shocks with all of the PZTs in a single plane were
taken in order to ensure response uniformity. Optimal
amplifier gains for each PZT were determined and set,
and the optimal threshold voltage used to trigger the
timing mechanisms was set to minimize uncertainty
in the velocity profile. PZT were then returned to their
axial locations.
Pressure was monitored throughout the experi-
ment with a PZT (Kistler Model 603B) at a location
10 mm from the endwall. Also located at 10 mm from
the endwall were observation windows for monitor-
ing the emission from CH* (using a narrow line filter
10 nm FWHM centered at 431 nm) and OH* (using a
Schott Glass UG-5 filter with > 95% transmission at
306 nm). A more complete description of the HPST
facility is given by Petersen et al. [10].
2.2. Available test time
In this study the experimental strategy to acquire
ignition time data was to measure low-pressure (2–
11 atm) ignition times in the large diameter (15.2 cm)
low-pressure shock tube and to measure high-pressure

3. 302 B.M. Gauthier et al. / Combustion and Flame 139 (2004) 300–311
Fig. 1. Available test time in the low-pressure shock tube
(LPST). Published 13 bar n-heptane/air ignition times from
the Aachen group shown in dashes.
(50 atm) ignition times in the smaller diameter
(5.0 cm) high-pressure shock tube. Before making
these measurements, a series of pure N2 experiments
at intermediate to low temperatures were performed
to confirm that there is sufficient test time in each
shock tube to measure ignition delay times under the
conditions of interest.
Available test time is defined as the time interval
between the arrival of the reflected shock at the obser-
vation port (10 mm from end wall for HPST, 20 mm
for the low-pressure shock tube) and the arrival at that
same location of a significant pressure disturbance
from the predicted or measured P5 (reflected shock
pressure). This disturbance usually develops from an
internal shock reflection from the contact surface or
from a rarefaction wave propagating from the end of
the driver section.
The available test time measurements for the low-
pressure, large diameter shock tube are shown in
Fig. 1. Measured n-heptane/air ignition times from
the Aachen group are also shown. Based on these
ignition time measurements, there appears to be suf-
ficient test time in the low-pressure shock tube to
measure n-heptane/air ignition times at temperatures
above 1000 K.
The available test time measurements for the
high-pressure, small diameter HPST are shown in
Fig. 2. Measured n-heptane/air ignition times from
the Aachen group are also shown. Based on these ig-
nition time measurements, there is sufficient test time
in the HPST to measure ignition times at all tempera-
tures of interest.
2.3. Measuring ignition delay times
The ignition delay time in this study is defined
as the time interval between the arrival of the re-
flected shock wave and the onset of ignition at the
Fig. 2. Available test time in the HPST. Published 42 bar
n-heptane/air ignition times from the Aachen group shown
in dashes.
sidewall observation location. The arrival of the re-
flected shockwave was determined by the step rise
in pressure at the observation location, and the on-
set of ignition was determined by monitoring either
the pressure history or the emitted light correspond-
ing to an intermediate species. The onset of ignition
from the pressure history (as well as both CH* and
OH* emission) was defined by locating the time of
steepest rise and linearly extrapolating back in time
to a preignition baseline (see Fig. 3). The three diag-
nostics generally agreed to within 5%. Another com-
monly used ignition delay time definition uses the
time to the first peak of a given diagnostic, and under
most conditions of this study gives nearly identical
results with the definition employed. The definition
based on the location of steepest rise, however, is a
more reliable definition for low-temperature ignitions
of gasoline or surrogate blends where a clear peak
cannot always be unequivocally identified. In the dis-
cussion that follows, all the results presented refer to
the pressure diagnostic and the definition involving
the back-extrapolation of the steepest rise; however,
the ignition time variance between the multiple defin-
itions has been included in the uncertainty analysis.
When using a sidewall diagnostic it is often impor-
tant to account for the effect of combustion wave ac-
celeration on the sidewall measurement. In the case of
highly exothermic mixtures, as the combustion wave
transitions to a detonation wave it can shorten the dis-
tance between the shock wave and the combustion
front, resulting in an erroneously short ignition time
as measured by a sidewall diagnostic (see [7,10]). The
significance of this sidewall error decreases with de-
creasing distance from the endwall, and with decreas-
ing temperature. For the present diagnostics located
10 mm from the endwall, and for reflected shock tem-
peratures less than 1300 K, the estimated discrepancy
is less than 10 µs for all cases, and has been included
in the uncertainty analysis.

4. B.M. Gauthier et al. / Combustion and Flame 139 (2004) 300–311 303
Fig. 3. Example gasoline/air ignition data showing ignition
delay time definition.
Table 1
Example of chemically frozen shock code predictions of re-
flected shock temperature and pressure, T5 and P5
Incident Ms F-F F-E E-E
3.34 T5 (K) 1681 1297 1301
P5 (atm) 12.32 11.50 13.25
2.98 T5 (K) 1382 1103 1105
P5 (atm) 46.69 43.84 49.27
F-F signifies vibrationally frozen incident and reflected
shock conditions; F-E, vibrationally frozen incident
and vibrationally equilibrated reflected conditions; E-E
vibrationally-equilibrated incident and reflected conditions.
E-E reflected shock pressures are larger than either F-F or
F-E pressures.
The overall uncertainty in the reflected shock tem-
perature was estimated to be 1.8% and was dominated
by fuel composition discrepancy and the uncertainty
in the endwall velocity. This corresponded to a maxi-
mum estimated uncertainty in the ignition delay time
measurements of approximately 15%.
2.4. Assumption of vibrational relaxation
In shock tube ignition time studies in air, care must
be taken to ensure that the vibrational relaxation of O2
and N2 is taken into consideration when computing
the reflected shock temperatures. In the present mix-
tures (1.87% n-heptane/air) reflected shock pressures
are strongly sensitive to the vibrational state of the
incident and reflected test gas. Example chemically
frozen reflected shock temperatures and pressures, T5
and P5, predictions and vibrational relaxation times
are given in Table 1. Large variations are evident in
the predicted temperatures and pressures in the re-
flected shock regime depending on the assumptions
made about the vibrational relaxation state of the test
gas.
Fig. 4. Comparison of measured and modeled reflected
shock pressures. All pressures are given relative to P5(E-E).
See Table 2 for definitions of E-E, F-E, and F-F. Measured
shock pressures agree with the (E-E) model, and not with the
(F-F) or (F-E) model.
We have compared the measured reflected shock
pressure for each shock wave experiment to the pre-
dicted reflected shock pressure given by the chemi-
cally frozen shock code. The results of this compar-
ison are shown in Fig. 4. Measured reflected shock
pressures are consistent with the assumption that the
test gas is vibrationally relaxed in both the incident
and the reflected regime. Temperatures and pressures
for this study, therefore, use the E-E (equilibrium
incident–equilibrium reflected) shock conditions.
2.5. Mixture preparation for liquid fuels
Mixtures were prepared in a 12.84-liter, mag-
netically stirred, stainless-steel mixing tank evacu-
ated using a mechanical pump. While mixtures in-
volving relatively high vapor pressure fuels (such as
n-heptane/air) can be made manometrically to a high
degree of accuracy, mixtures with low vapor pressure
components require an alternative method.
Liquid mixture components were measured vol-
umetrically using glass burettes and drawn into the
evacuated and heated mixing tank through a heated
manifold designed to accommodate low vapor pres-
sure fuels. After the pressure had stabilized from the
evaporation of the liquid component, the tank pres-
sure was measured (MKS Inc. Baratron) and com-
pared to the theoretical pressure predicted from the
ideal gas law. The discrepancy between the two, likely
due to adsorption to surface sites and volumetric
measurement uncertainty, was occasionally substan-
tial and was included in the uncertainty analysis. For
the entire study, air refers to synthetic (dry) air con-
sisting of 21% O2 and 79% N2. Research grade gases
(Praxair N2, O2, and CO2) were added slowly in ac-
cordance with a procedure outlined by Horning et al.

5. 304 B.M. Gauthier et al. / Combustion and Flame 139 (2004) 300–311
[7] to inhibit condensation of fuel that could lead to
inaccuracies in reported fuel composition. Each mix-
ture was allowed to mix for a minimum of 3 h before
use.
Care was taken to ensure that at all times the va-
por partial pressure of each component remained be-
low the saturation pressure for the given temperature.
For these particular experiments, in order to realize
a practical and time-efficient mixture yield (at least
four experiments per mixture), the entire tank was
insulated and heated to approximately 60 ◦C. While
higher temperatures would be even more advanta-
geous, a precursory gas chromatograph study indi-
cated that in the time scales necessary for proper
mixing and to use the mixture, chemical cracking
of the fuel components became noticeably significant
at temperatures above 110 ◦C. A lower mixture stor-
age temperature and a shorter time necessary to use
a smaller mixture lessen the likelihood of fuel alter-
ation. Tests with a gas chromatograph confirmed this,
as no distinguishable fuel alterations were observed
for practical time scales at 60 ◦C.
The spatial temperature uniformity of the tank is
also significant as it can be important when consid-
ering local condensation points or when applying the
ideal gas law to determine the necessary quantity of
fuel. Although the mixing tank was well insulated,
the surface temperature distribution from the use of
heating tape on the outside of the tank allows for the
possibility of thermal gradients on the inside of the
tank. A conservative estimate of the spatial temper-
ature uniformity was estimated to be within 10 ◦C
(∼ 3%) inside of the tank. The effect on the reported
fuel concentration was included in the uncertainty
analysis, and to minimize the effect of this on possi-
ble condensation, the maximum mixing tank pressure
was designed to keep partial pressures of all compo-
nents to less than 3/4 of the saturated vapor pressure
at the corresponding temperature (tank temperature of
∼ 60 ◦C). A similar consideration was made to inhibit
condensation when filling the shock tube (tempera-
ture ∼ 25 ◦C).
As a check to ensure that the proper fuel compo-
sition is reported, the measured pressure behind the
reflected shock wave (P5) was compared with the
expected pressure from the normal shock equations,
and with the sole exception of the high concentra-
tion (Φ = 2.0) gasoline mixture, all experiments sup-
ported fuel concentration discrepancies well within
the estimated uncertainty of 10%. For the high con-
centration (Φ = 2.0) gasoline mixtures, the postex-
perimental analysis indicated that initial condensation
of fuel resulted in a fuel concentration that increased
with each experiment in the following sequence (Φ =
1.6, 1.8, 1.9, 2.1). No change in Φ was noticed in any
other mixture, which further supports the confidence
Table 2
RD387 gasoline composition (major components by mol%)
Component name mol%
Cyclopentane 16.8
Toluene 9.7
Isopentane 7.8
meta-Xylene 4.9
3-Methylhexane 4.4
n-Heptane 3.6
2-Methylhexane 3.3
Ethylbenzene 3.2
n-Pentane 3.0
2,2,4-Trimethylpentane (iso-octane) 2.5
in reported fuel composition for the remainder of the
experiments.
The two ternary surrogates used in the igni-
tion time experiments consisted of mixtures of iso-
octane, toluene, and n-heptane in the following pro-
portions: surrogate A [63/20/17%] and surrogate B
[69/14/17%] by liquid volume, or surrogate A [56/
28/17%] and surrogate B [63/20/17%] by mole frac-
tion. The n-heptane, iso-octane, and toluene used in
the ternary surrogates were each research grade qual-
ity and supplied by Sigma-Aldrich. RD387 gasoline
was supplied by General Motors Research & Devel-
opment and Planning. This is an 87 Octane Num-
ber, (RON + MON)/2, gasoline with an H/C ratio
of 1.85 blended to represent a “customer average”
regular-grade reformulated gasoline without added
oxygenates. The major components of RD387 are
listed in Table 2. For experiments involving exhaust
gas recirculation (EGR), the EGR ratio is defined such
that a mixture with X% EGR corresponds to a mix-
ture of (100 − X) mol% of the fuel/air mixture and
X mol% of the products that result from the com-
plete conversion of the fuel/air mixture to CO2, H2O,
O2, and N2. For these cases, distilled water was used
and treated in the same manner as the liquid fuels de-
scribed above.
2.6. Thermodynamic surrogate for gasoline
A three-component thermodynamic surrogate was
used to approximate gasoline in the calculations of the
reflected shock conditions for this fuel. The surrogate
composition (different from the surrogate mixtures
used in the ignition time experiments) was chosen
to best match the average molecular weight and the
specific heats at 300 and 1000 K of the gasoline sam-
ple: benzene 56%, n-pentane 10%, iso-octane 34% by
mole fraction. A comparison between the measured
reflected shock pressure and the pressure calculated
with the thermodynamic surrogate showed excellent
agreement.

6. B.M. Gauthier et al. / Combustion and Flame 139 (2004) 300–311 305
3. Experimental results
The results of the ignition delay time experiments
for n-heptane, gasoline, surrogate A, and surrogate
B are presented in Table 3 (low-pressure shock tube)
and Table 4 (HPST). The results are presented in the
following order: Section 3.1, pressure scaling of data;
Section 3.2, n-heptane ignition time measurements;
Section 3.3, gasoline and surrogate measurements for
Φ = 1; Section 3.4, effect of variation of equivalence
ratio; and Section 3.5, effect of addition of EGR.
3.1. Pressure scaling
Shock tubes can reproduce near, but not identi-
cal, pressures and temperatures from shock experi-
ment to shock experiment. As well, for similar di-
aphragm thicknesses, the reflected shock pressure
drops slightly as reflected shock temperature goes up.
For a uniform graphic presentation of the data, a pres-
sure scaling of all the data points in a similar pressure
regime is needed. Generally this is done by perform-
ing experiments over a wider range of pressures and
assuming a power-law dependence to the pressure
scaling. In the following discussion, we apply this
method and scaling to the n-heptane, gasoline and
surrogate ignition time data. It should be noted that
while this normalization is useful in a limited temper-
ature and pressure regime, the temperature and pres-
sure variations in ignition delay times are complicated
enough that simple power-law pressure dependencies
do not capture the complete picture of this ignition
behavior.
Experiments of n-heptane were performed in the
temperature range of 900–1400 K, and at pressures
near 2, 10, 20, and 55 atm. Gasoline and surrogate
mixtures in air were performed in the range of 850–
1250 K, and at pressures near 20 and 55 atm. The
variation of the ignition time with pressure was de-
Table 3
Ignition delay time measurements for the low-pressure
shock tube
Φ EGR (%) T5 (K) 1000/T (1/K) P5 (atm) τign (µs)
n-Heptane 2 atm
1.0 0 1249 0.801 1.97 529
1.0 0 1299 0.770 1.89 311
1.0 0 1358 0.736 1.85 152
1.0 0 1378 0.726 1.99 117
n-Heptane 10–12 atm
1.0 0 1305 0.766 10.62 117
1.0 0 1236 0.809 11.24 207
1.0 0 1299 0.770 10.25 122
1.0 0 1344 0.744 10.27 85
1.0 0 1290 0.775 11.88 118
Fig. 5. Comparison of n-heptane Φ = 1 ignition delay
times at 12, 20, 55 atm (current study) and 13 and 41 atm
(Aachen). There is strong evidence of an NTC region occur-
ring at higher temperatures for higher pressures. Each data
set is fit with a quadratic polynomial.
termined for each fuel and was assumed to vary as
P−N .
For the low-pressure n-heptane experiments (2–
10 atm) the pressure scaling of P−0.55 was adopted
from a similar study by Horning et al. [7] over a
similar range of conditions (1–6 atm, 1330–1620 K).
For the high-pressure n-heptane experiments the data
from the present study and the data from the Aachen
group [3–5] have been combined. No pressure scaling
factors have been reported for the earlier work from
Aachen. A comparison of ignition time data from five
different data sets, 12, 20, and 55 atm from Stanford,
and 13, 41 atm from Aachen, reveals a substantial ig-
nition time dependence on pressure (Fig. 5).
The two sets of data are in excellent agreement.
In each case, the ignition time is seen to increase
with decreasing temperature until reaching a rela-
tive maximum, and then decrease with further de-
crease in temperature (negative temperature depen-
dence). Quadratic polynomial curves were fit to the
data to capture the general shapes of each dataset.
The quadratic curves fit the data well for the 20, 41,
and 55 atm sets; however, a cluster of 13 atm data
near 900 K from the Aachen group appears to differ
from the consensus trend. In this case, the quadratic
curve for the 13 atm data is fit to the more consis-
tent measurements at the higher and lower tempera-
tures.
The curves indicate that along with power-law
pressure dependence, there must be a coupled temper-
ature dependence to account for the temperature dif-
ferences at each peak, as well as the relative breadths
of the curves. The pressure scaling is expected to have
a dependence on temperature, but in order to simplify
the analysis, the coupled temperature dependence was
neglected, and the power-law pressure dependence

7. 306 B.M. Gauthier et al. / Combustion and Flame 139 (2004) 300–311
Table 4
Ignition delay time measurements from the HPST
Φ EGR
(%)
T5
(K)
1000/T
(1/K)
P5
(atm)
τign
(µs)
Φ EGR
(%)
T5
(K)
1000/T
(1/K)
P5
(atm)
τign
(µs)
n-Heptane 15–25 atm n-Heptane 45–60 atm
1 0 806 1.241 20.0 1377 1 0 909 1.100 53.9 254
1 0 850 1.176 19.9 1653 1 0 923 1.083 60.0 244
1 0 906 1.104 19.8 1836 1 0 926 1.080 60.6 233
1 0 1012 0.988 18.1 944 1 0 932 1.073 55.4 323
1 0 1048 0.954 16.7 854 1 0 985 1.015 48.6 364
1 0 1007 0.993 57.7 232
Gasoline 15–25 atm 1 0 1013 0.987 53.6 292
1.0 0 986 1.014 23.6 1317 1 0 1023 0.978 54.2 261
1.0 0 1011 0.989 18.7 1395 1 0 1027 0.974 59.1 237
1.0 0 1048 0.954 21.6 908 1 0 1057 0.946 50.0 194
1.0 0 1053 0.950 16.5 1068 1 0 1063 0.941 53.1 179
1.0 0 1153 0.867 23.6 208 1 0 1115 0.897 52.3 102
1.0 0 1158 0.864 17.3 271
1.0 0 1280 0.781 15.4 51 Gasoline 45–60 atm
1.0 20 1014 0.986 17.9 1651 1.0 0 902 1.109 53.4 1558
1.0 20 1048 0.954 18.0 1122 1.0 0 903 1.107 48.1 1299
1.0 20 1081 0.925 17.9 759 1.0 0 929 1.076 54.4 1265
1.0 20 1191 0.840 15.6 241 1.0 0 947 1.056 60.9 890
1.0 30 1042 0.960 18.1 1321 1.0 0 977 1.024 54.0 734
1.0 30 1081 0.925 18.0 841 1.0 0 1015 0.985 54.6 465
1.0 30 1143 0.875 18.3 421 1.0 0 1045 0.957 47.8 355
1.0 30 1160 0.862 16.3 390 1.0 0 1083 0.923 48.6 229
0.5 0 1031 0.970 16.1 1805 0.5 0 988 1.012 49.5 1243
0.5 0 1125 0.889 15.3 661 0.5 0 1111 0.900 45.0 281
0.5 20 1067 0.937 15.8 1445 0.5 20 1091 0.917 46.9 463
0.5 20 1227 0.815 15.5 186 0.5 20 1115 0.897 51.1 341
1.9 0 1023 0.978 16.5 1367 1.6 0 916 1.092 55.2 928
2.1 0 1144 0.874 14.9 411 1.8 0 1048 0.954 53.0 233
Surrogate A 15–25 atm Surrogate A 45–60 atm
1.0 0 1023 0.978 16.6 1288 1.0 0 859 1.164 48.7 1428
1.0 0 1035 0.966 17.4 1041 1.0 0 930 1.075 52.1 1198
1.0 0 1065 0.939 19.6 746 1.0 0 1049 0.953 51.8 376
1.0 0 1116 0.896 15.9 550 0.5 0 992 1.008 54.3 1122
1.0 0 1118 0.894 18.4 443 0.5 0 1021 0.979 49.7 893
1.0 0 1137 0.880 19.4 268 0.5 0 1138 0.879 53.7 185
1.0 20 1051 0.951 16.8 1233 0.5 20 1053 0.950 51.4 696
1.0 20 1080 0.926 16.4 914 2.0 0 840 1.190 53.7 911
1.0 20 1131 0.884 14.6 568 2.0 0 974 1.027 54.3 576
1.0 20 1157 0.864 14.6 381 2.0 0 1054 0.949 48.3 236
1.0 30 1036 0.965 17.5 1754
1.0 30 1085 0.922 17.4 949 Surrogate B 45–60 atm
1.0 30 1104 0.906 14.7 869 1.0 0 907 1.103 53.4 1152
0.5 0 1059 0.944 18.3 1326 1.0 0 914 1.094 58.3 918
0.5 0 1183 0.845 16.9 252 1.0 0 947 1.056 56.3 866
0.5 20 1068 0.936 16.0 1348 1.0 0 980 1.020 49.2 767
0.5 20 1195 0.837 14.3 263 1.0 0 985 1.015 58.9 637
2.0 0 1049 0.953 16.1 1003 1.0 0 1005 0.995 53.7 537
2.0 0 1214 0.824 14.2 126 1.0 0 1011 0.989 50.0 569
1.0 0 1065 0.939 49.7 297
Surrogate B 15–25 atm 1.0 0 1092 0.916 55.1 182
1.0 0 972 1.029 24.6 1528 1.0 0 1103 0.907 49.3 180
1.0 0 1101 0.908 24.8 368
1.0 0 1136 0.880 25.0 237

8. B.M. Gauthier et al. / Combustion and Flame 139 (2004) 300–311 307
Fig. 6. Determination of pressure-scaling factor for
n-heptane from quadratic peaks. Strong linear agreement is
seen with a pressure scaling of P −1.64. Linear fit to peaks:
τ = 2.4 × 105 P −1.64 (µs).
was extracted by comparing the values of the peaks
of the quadratic fits to the ignition times (Fig. 6).
As shown by the linearity of data points in Fig. 6,
the ignition time peaks scale well with a power-law
pressure dependence of P−1.64. This is substantially
different from the scaling reported by Horning et al.
[7] over a higher temperature, lower pressure range
(1–6 atm, 1330–1620 K), which was determined to
be P−0.55. The appreciable difference in the pressure
scaling for n-heptane reflects the differences in the
high-temperature and low-temperature (NTC) chem-
istry.
Determining the pressure scaling factors for gaso-
line and the surrogate blends was simpler (in part
because there was less data and the subtle coupling
between temperature and pressure was not evident
as in n-heptane); for each mixture (gasoline, surro-
gate A and B) the experimental data collapsed onto a
slightly different quadratic polynomial when plotted
as (log(τ) vs 1/T ) (see Figs. 7–9). The pressure-
scaling factors calculated from these fits were for
gasoline, P−1.05; for surrogate, A P−0.83; and for
surrogate B, P−0.96. It would be more exact to state
that these pressure dependencies for ignition delay
times reflect the scaling for the two pressure regimes
studied, 20 and 50 atm, and for the intermediate tem-
peratures, 1000–1100 K, where the majority of the 20
and 50 atm data overlapped.
3.2. Ignition time measurements of n-heptane/air
Stoichiometric (Φ = 1.0) n-heptane/air ignition
time experiments were performed in the low-pressure
shock tube near 2 and 11 atm at temperatures above
1200 K. Ignition times were determined using PZT
pressure and CH emission measurements. The results
are shown in Fig. 10. For comparison with the Aachen
Fig. 7. Gasoline/air ignition delay times. Φ = 1.0,
15–60 atm, scaled as P −1.05 (shown with quadratic fit).
Fig. 8. Surrogate A/air ignition delay times. Φ = 1.0,
15–60 atm, scaled as P −0.83 (shown with quadratic fit).
Fig. 9. Surrogate B/air ignition delay times. Φ = 1.0,
15–60 atm, scaled as P −0.96 (shown with quadratic fit).
group data, all the data were normalized to 13 bar
using a pressure scaling of P−0.55 based on the corre-
lations of Horning et al. [7]. Correlated in this manner,
there is good agreement between the higher tempera-

9. 308 B.M. Gauthier et al. / Combustion and Flame 139 (2004) 300–311
Fig. 10. n-Heptane/air ignition delay times for the
low-pressure regime. Φ = 1.0.
Fig. 11. Comparison of measured ignition delay times for
n-heptane/air for the high-pressure regime. Φ = 1.0.
ture 2 and 11 atm data and the extrapolation of the
Aachen data.
The results of the (Φ = 1.0) n-heptane experi-
ments, for temperatures below 1200 K in both the
15–20 and 45–60 atm range are shown in Fig. 11
with the results of similar experiments performed by
Fieweger et al. A comparison indicates a consensus
between the current study and the Aachen studies, es-
pecially at the higher temperatures. As noted in the
previous section, at temperatures around 900 K, there
appears to be a cluster of points in the 12.8 atm data
reported by Fieweger et al. that do not agree with the
consensus. The evidence for strong NTC behavior at
13 and 42 atm reported by the Aachen group is sup-
ported by the present measurements at 20 and 55 atm.
3.3. Gasoline and ternary surrogates, Φ = 1.0
The results of the Φ = 1.0 gasoline, surrogate A
and surrogate B ignition delay time experiments in
the low-pressure regime are shown in Fig. 12 with the
Fig. 12. Ignition delay times for gasoline, surrogate A, and
surrogate B, Φ = 1.0, 15–25 atm. Gasoline data scaled to
20 atm as P −1.05, surrogate A as P −0.83, and surrogate B
as P −0.96. Solid line, quadratic fit to gasoline data.
Fig. 13. Ignition delay times for gasoline, surrogate A, and
surrogate B, Φ = 1.0, 45–60 atm. Gasoline data scaled to
55 atm as P −1.05, surrogate A as P −0.83, and surrogate B
as P −0.96. Solid line, quadratic fit to gasoline data.
measurements scaled to 20 atm with their respective
scaling factors. The measured ignition times of the
two surrogates are effectively the same over this tem-
perature range, and match the ignition delay times for
gasoline very well.
The results of the high-pressure measurements are
shown in Fig. 13 with the measurements scaled to
55 atm with their respective scaling factors. As in the
15–20 atm experiments, the measured ignition times
of the two surrogates are indistinguishable over this
temperature range, and match the ignition delay times
for gasoline very well.
Slight NTC behavior is seen in both the low-
and high-pressure ignition times evidenced by the
quadratic roll-off behavior of the ignition time at low
temperatures.

10. B.M. Gauthier et al. / Combustion and Flame 139 (2004) 300–311 309
Fig. 14. Ignition delay times for gasoline (Φ = 0.5, Φ = 1.0,
Φ = 2.0) in the 15–25 atm range. Shown with quadratic fit to
Φ = 1.0 gasoline data and a similar fit to the Φ = 0.5 data.
No fit is attempted for Φ = 2.0 data, where no significant
variation from the Φ = 1.0 data was seen.
Fig. 15. Ignition delay times for gasoline (Φ = 0.5, Φ = 1.0,
Φ = 2.0) in 45–60 atm range. Shown with quadratic fit to
Φ = 1.0 gasoline data and similar fits to the Φ = 0.5 and
Φ = 2.0 data. Unlike in 15–25 atm data, a noticeable trend
is observed in the Φ = 2.0 data.
3.4. Effect of variation of equivalence ratio
Gasoline experiments with equivalence ratios of
Φ = 1.0, Φ = 0.5, and Φ = 2.0 at low and high pres-
sures are compared in Figs. 14 and 15, respectively.
Surrogate A experiments with equivalence ratios of
Φ = 1.0, Φ = 0.5, and Φ = 2.0 at low and high pres-
sures are compared in Figs. 16 and 17, respectively.
A clear trend of longer ignition delay times for the
lean (Φ = 0.5) as compared to the Φ = 1.0 exper-
iments is noted. For richer (Φ = 2.0) mixtures the
effect on ignition delay time is insignificant at low
pressures, but is clearly evident in the higher pressure
data for both gasoline and surrogate A.
A direct comparison between the gasoline and the
surrogate A data for both Φ = 0.5 and 2.0 and at both
low and high pressures shows good agreement, the
Fig. 16. Ignition delay times for surrogate A (Φ = 0.5,
Φ = 1.0, Φ = 2.0) in 15–25 atm range. Shown with
quadratic fit to Φ = 1.0 gasoline data and a similar fit to the
Φ = 0.5 data. No fit is attempted for Φ = 2.0 data, where no
significant variation from the Φ = 1.0 data was seen.
Fig. 17. Ignition delay times for surrogate A (Φ = 0.5,
Φ = 1.0, Φ = 2.0) in 45–60 atm range. Shown with
quadratic fit to Φ = 1.0 gasoline data and similar fits to the
Φ = 0.5 and Φ = 2.0 data. Unlike in 15–25 atm data, a no-
ticeable trend is observed in the Φ = 2.0 data.
gasoline and surrogate A data being effectively indis-
tinguishable.
3.5. Exhaust gas recirculation
In order to avoid condensation of water at the ini-
tial pressures necessary for these experiments, the ef-
fect of exhaust gas recirculation was studied in the
15–20 atm range, with a few select conditions in
the 45–60 atm range where condensation could be
avoided. Shown below in Figs. 18 and 19 are the re-
sults of the 15–25 atm, Φ = 1.0, EGR experiments
(including baseline 0% EGR) for gasoline and surro-
gate A, respectively.
Based on linear fits to the experimental data in the
regions of interest, the effect of exhaust gas recircu-

11. 310 B.M. Gauthier et al. / Combustion and Flame 139 (2004) 300–311
Fig. 18. Ignition delay times for gasoline, Φ = 1.0,
15–25 atm range, for EGR ratios of 0, 20, and 30%. Shown
with linear fits over the temperature regions of interest to
capture the effect of EGR.
Fig. 19. Ignition delay times for surrogate A, Φ = 1.0,
15–25 atm range, for EGR ratios of 0, 20, and 30%. Shown
with linear fits over the temperature regions of interest to
capture the effect of EGR.
lation on the ignition delay times is significant and
similar in trend for both the gasoline and the surro-
gate A experiments. However, the magnitude of the
effect appears slightly larger for surrogate A than for
gasoline, and this increase is greatest at the lowest
temperatures.
For the Φ = 0.5 experiments, however, there ap-
pears to be no discernible difference between the ex-
periments with and without EGR (Fig. 20) for gaso-
line and surrogate A.
4. Conclusions
Ignition times of n-heptane, gasoline, and two sur-
rogate fuels in air have been measured in a high-
pressure shock tube under conditions similar to those
Fig. 20. Ignition delay times for gasoline and surrogate A,
Φ = 0.5, 15–25 atm range, for EGR ratios of 0 and 20%. No
distinction is seen between 0 and 20% EGR loading exper-
iments. As in previous comparisons, gasoline and surrogate
agree well.
found in HCCI engine cycles. The n-heptane study
is in good agreement with the shock tube study of
Fieweger et al. and supports the observation of a pro-
nounced, low-temperature, NTC region. Ignition de-
lay times for RD387 gasoline/air were successfully
reproduced by ternary surrogate mixtures comprised
of iso-octane, toluene, and n-heptane (in air) for tem-
peratures between 850 and 1250 K, pressures between
15 and 60 atm, stoichiometric, lean, and rich equiva-
lence ratios (Φ = 1.0, 0.5, 2.0), and exhaust gas recir-
culation loadings from 0 to 30%. The two surrogate
fuels maintained a common fractional composition of
n-heptane and varied the toluene/iso-octane composi-
tion. No noticeable difference between the two surro-
gates was observed by the ignition delay times. Strong
agreement was seen between surrogate A and RD387
gasoline over all experimental conditions. Surrogate
B was only studied in the nominal, Φ = 1.0, experi-
mental range and also showed strong agreement with
RD387 gasoline. In fuel lean mixtures (Φ = 0.5), the
ignition times were longer than the Φ = 1.0 mix-
tures for both gasoline and surrogate A at both low-
(15–25 atm) and high- (45–60 atm) pressure condi-
tions. Fuel-rich (Φ = 2.0) conditions for both gaso-
line and surrogate A were indistinguishable in the
low-pressure (15–25 atm) range, but were clearly dis-
tinct under the high-pressure (45–60 atm) conditions.
Exhaust gas recirculation loading affected the ig-
nition delay times for Φ = 1.0 experiments similarly
for both gasoline and surrogate A, with clear trends
indicating an increase in ignition delay time with in-
creased EGR loading. For Φ = 0.5 experiments, how-
ever, no discernible differences between experiments
with 0 and 20% EGR were seen for gasoline and sur-
rogate A.

12. B.M. Gauthier et al. / Combustion and Flame 139 (2004) 300–311 311
Experiments using the surrogate mixtures suc-
cessfully matched the ignition delay time measure-
ments found in gasoline, but we should reiterate that
these particular surrogate mixtures may not dupli-
cate gasoline mixture results in other engine experi-
ments.
Ignition delay times are useful targets for valida-
tion and refinement of reaction mechanisms. They
provide the needed data to examine the effects of
fuel composition, mixture stoichiometry, and EGR
dilution on the ignition process. They are, how-
ever, only one set of targets. Other constraints on
the internal structure of reaction mechanisms can
be determined from pressure measurements, such as
preignition energy release, and species concentration
profiles, which provide information about the tran-
sient radical pool. Measurements of these targets are
needed.
Acknowledgment
The current work was supported by the General
Motors Research and Development Center, Warren,
Michigan.
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