International Journal of Engineering Trends and Technology (IJETT) – Volume 29 Number 6 - November 2015
Ignition Mechanism Analysed through Transient Species
Measurements and its Correlation with 0-D and 3-D
Simulations for PRF and Toluene/ n-heptane Mixture
Mohd Adnin Bin Hamidi#1#2 Muhammad Faizzuddeen Bin Yunadi#1 Atsumu Tezaki#1
#1 University of Toyama, 3190 Gofuku, Toyama 930-8555, JAPAN
#2 University Malaysia Pahang, MALAYSIA
1
adninaddin@gmail.com
2
kembar.albiruni@gmail.com
3
tezaki@eng.u-toyamna.ac.jp
Abstract— FT-IR was used to detect intermediate
species which formed throughout the exhaust gas in
a test engine at cool ignition condition. PRF (isooctane/n-heptane) and NTF (toluene/n-heptane)
were used as fuel mixtures. Fuel consumption
decreases with increasing iso-octane content in PRF
and toluene content in NTF. These tendencies
correspond to the difference in detail reaction
mechanism for PRF and NTF oxidation. The
essential mechanism affecting the ignition property
of n-heptane is discussed by simplified model
supported by simulation by Chemkin Pro
reproducing the experimental results. FORTE was
used to analyze temperature and fuel concentration
change inside combustion chamber through multidimensional simulation.
Keywords— Homogenous Charge Compression
Ignition / PRF, NTF, Low Temperature Oxidation,
FT-IR, Chemical Kinetics.
I. INTRODUCTION
A typical process of compression ignition is
composed of low and high temperature oxidation
stages. Ignition timing of the high temperature
oxidation is influenced by heat release and
composition change in the low temperature
oxidation (LTO). While high temperature oxidation
mechanism is common to most hydrocarbons, LTO
property depends strongly on molecular structure of
hydrocarbons.
Our
group
has
conducted
experimental methodologies which is originated
from a study of Leppard et al. [2], can detect variety
of species present after LTO.
In this study, we applied the methodologies to
NTF (n-heptane and toluene mixture). Toluene and
iso-octane are typical high octane number fuels that
reduce ignition activity of n-heptane when mixed,
but some different behaviors between these fuels on
the ignition control were reported by Shibata et al.
[3]. In the following chapters, we present data of
chemical composition in the course of compression
ignition of NTF. Relevant chemical kinetic
mechanisms are discussed with a simplified model
constructed with a consideration of the property of
chain reaction. Finally the ignition behaviour
ISSN: 2231-5381
changing with the fuel mixing ratio is reproduced by
non-dimensional and 3-dimensional numerical
simulations by using CHEMKIN PRO and FORTE,
respectively.
II. EXPERIMENTAL SETUP
As shown in fig. 1, the experimental engine is a
single cylinder, 4-stroke overhead valve engine
(displacement 541 cm3, compression ratio 8.0). It is
motored with the standard speed of 600 rpm. Liquid
fuel is injected into the intake port, evaporated and
mixed with preheated air to form combustible
mixture gas. Crank angle resolved cylinder pressure
is measured by a mounted pressure gauge. For
Exhaust Gas Analysis, a part of exhaust gas is
introduced in an optical cell of 3 m path length at the
pressure of 10 kPa, and the chemical composition is
analyzed by a Fourier transform Infrared
spectrometer (FT-IR, Shimadzu IR Prestige-21).
Fuel mixtures are prepared prior to experiments. The
mixing composition is defined by liquid volume
ratio and indicated by the percentage of toluene for
NTF; for example, NTF10 is 10% toluene + 90% nheptane mixture.
Non dimensional and 3-dimensional numerical
simulation of compression ignition was carried out
on CHEMKIN PRO and FORTE platform with
existing detailed oxidation reaction models of M.
Mehl for PRF and NTF [4], [5]. (Size 10 &
Normal)An easy way to comply with the conference
paper formatting requirements is to use this
document as a template and simply type your text
into it.
III. RESULTS AND DISCUSSIONS
A. Composition Change in Cool Ignition Stage
Figure 2 shows species composition after low
temperature ignition stage depending on toluene
content in NTF. This is a result of model calculation
described in the next section, and is in good
agreement with experimental observation. Overall
trend is that the fuel consumption and formaldehyde
production decrease with increasing toluene content;
however, the decreasing rate is slow at lower toluene
content up to 60% and turns fast until they at over
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International Journal of Engineering Trends and Technology (IJETT) – Volume 29 Number 6 - November 2015
60% until the consumption and production fall to
zero at around 80% of toluene.
Where ki is the rate constant of species i with OH,
and the subscript a denotes aldehyde.
Above equations can be translated into a form in
which the independent variable is percentage
consumption of base fuel x as:
(7)
(8)
(9)
B. Simplified Model
The low temperature oxidation mechanism of
hydrocarbons consists of complicated reactions such
as H-abstraction, O2-addition, isomerization, second
O2-addition, decomposition and branching passes.
Here, the oxidation mechanism of a mixture of nheptane (base fuel) and toluene (second fuel) is
expressed in a summarized manner as:
nC7H16 + OH  bOH + bHCHO + other products
(1)
C6H5CH3 + OH  sOH + sHCHO + other
products
(2)
where αi is the reproduction index of OH from
species i, and βi is the production yield of chain
terminating intermediate represented by HCHO from
species i. There are products that absorb OH other
than HCHO, but they are included in the group of
“HCHO”. Other products are regarded as unreactive
during the course of low temperature reaction.
Differential equations governing time evolution of
relevant species are described as:
=
,
(3)
=
,
(4)
production or consumption
Fig.1 Experimental apparatus for exhaust gas
analysis.
where ys is the remaining amount of second fuel
(relative to initial amount of base fuel, and so forth),
ya is the accumulated amount of aldehyde, yOH is the
amount of OH, g1 = ks/kb and g2 = ka/kb. αb of low
ON fuel is necessarily exceeding unity, but αs may
differ. When initial overall of OH reproduction
index is over unity, the OH concentration increases
with repetition of the reaction chain. According to
decrease in fuel and increase in the OH consuming
aldehyde, the slope OH increase gradually reduces,
turns into decrease, and finally the chain system is
terminated. The point of termination is represented
by the overall OH reproduction index = 1.The
parameters used for NTF are kｂ/ks = 5, αb= 2, αs= 0,
β ｂ = 1.6, βs= 0, a part of which was assumed by
overlooking the toluene oxidation mechanism [6].
The composition at the point of termination is
summarized in Figs. 2. These calculations
successfully reproduce the experimentally observed
tendency, i.e., toluene consumption is less than those
of n-heptane, the effect of reducing n-heptane
consumption is less in toluene, and aldehyde
production reduces with increasing toluene content.
1
n-heptane
Toluene
[HCHO]/[Fuel] 0
0.8
0.6
0.4
0.2
0
0
20
40
60
80
toluene content [%]
Fig.2 Model results of fuel consumption and
formaldehyde production at the end of lowtemperature chain reaction in NTF compression as a
function of toluene content.
=
(5)
=
(6)
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C. Simulation of Ignition Processes
Figure 3 shows comparison of temperature
profiles between CHEMKIN and FORTE, in which
average in the cylinder is taken for FORTE. Fuel is
n-heptane with 0.5 equivalence ratio. For FORTE
calculation, 403.5K was used as intake temperature
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International Journal of Engineering Trends and Technology (IJETT) – Volume 29 Number 6 - November 2015
to simulate the real experimental work with our
engine. The CHEMKIN calculation result of 403.5K
intake shows distinctive difference from FORTE
calculation, and the best matching was obtained at
550K intake temperature of CHEMKIN. There may
be a number of reasons for this difference, such as
dimensionality of the calculation, heat transfer from
intake port and cylinder wall included only in
FORTE, and the difference in intake valve closing in
FORTE and starting crank angle in CHEMKIN. We
conclude the last reason is most important and the
other ones are minor.
Figure 4 shows cool stage and final ignition
timing as a function of toluene content in NTF,
obtained by FORTE. Low temperature heat release
(LTHR) of cool ignition stage arrives in the lower
range of toluene content. Since LTHR advances the
final hot ignition, there is a large difference in
ignition timing between the ranges of LTHR and no
LTHR. The trend is consistent with the experimental
and modeling observation of composition change
during the low temperature oxidation.
Fuel consumption decreases with increasing
toluene/n-heptane ratio although it’s weak. It is
because toluene has lower rate constant with OH, so
that toluene barely interrupts the OH reproducing
chain reaction of n-heptane.
Comparison between CHEMKIN and FORTE
calculation shows distinctive difference due to the
difference in dimensionality of the calculation, heat
transfer from intake port and cylinder wall included
only in FORTE, and the difference in intake valve
closing in FORTE and starting crank angle in
CHEMKIN. The trend in ignition timing in FORTE
calculation is consistent with the experimental and
modeling observation of composition change during
the low temperature oxidation.
Last but not least, further recalibration in
simulation work are needed to get more reliable in
comparing variable parameters.
ACKNOWLEDGMENT
This work was supported by JSPS KAKENHI
Grant Number 24560226. Mohd Adnin expresses
special thanks to University Malaysia Pahang,
UMP as the sponsor for his studies in University of
Toyama.
REFERENCES
[1]
[2]
[3]
Fig. 3 Temperature Profiles in Simulation Platforms.
[4]
[5]
[6]
A. Tezaki, T. Miyashita and H. Murasawa, Transient
Chemical Composition Analysis in HCCI of n-Heptane
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W. R. Leppard, A Comparison of Olefin and Paraffin
Autoignition Chemistries: A Motored-Engine Study, SAE
892081.
G. Shibata and T. Urushihara, Dual Phase High
Temperature Heat Release Combustion, SAE 2008-010007.
M. Mehl, H. J. Curran, W. J. Pitz and C. K. Westbrook,
Chemical
kinetic modeling of component mixtures
relevant to gasoline, European Combustion Meeting,
Vienna, Austria, 2009.
Mehl M., W.J. Pitz, C.K. Westbrook, H.J. Curran, Kinetic
Modeling of Gasoline Surrogate Components and Mixtures
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Fig.4 Ignition Timing vs. Toluene Percentage in
NTF, obtained by FORTE.
IV. CONCLUSIONS
Production yield of HCHO decreases with
increasing toluene/n-heptane ratio. HCHO is a
product of ketoalkylperoxide that makes benzyl
radical from toluene does not proceed such way
because of the firmness of its aromatic ring.
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