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Charge stratification to control HCCI: Experiments and CFD modeling
with n-heptane as fuel
Zhaolei Zheng, Mingfa Yao *
State Key Laboratory of Engines, Tianjin University, Weijin, Tianjin 300072, China
a r t i c l e i n f o
Article history:
Received 27 February 2008Received in revised form 30 June 2008
Accepted 4 September 2008
Available online 1 October 2008
Keywords:
Homogeneous charge compression ignition
(HCCI)
Charge stratification combustion
Multi-dimensional computational fluid
mechanics (CFD)
Chemical kinetics
a b s t r a c t
An optimized reduced mechanism of n-heptane including 42 species and 58 elementary reactions
adapted to charge stratification combustion is developed first in this study. Some engine experiments
and a fully coupled CFD and reduced chemical kinetics model with n-heptane as fuel are adopted to
investigate the combustion processes of HCCI-like charge stratification combustion aimed at diesel HCCI
application. For premixed/direct-injected stratification combustion, the low temperature reaction occurs
in the regions with homogeneous fuel first and high temperature reaction begins from high fuel concen-
tration regions involved in the spray process. With the increase of the injection ratio, the high tempera-
ture reaction occurs in advance, the pressure rise rate reduces, UHC emissions decrease and CO emissions
increase. At larger injection ratio, the onset of the high temperature reaction advances and the maximum
pressure rise rate decreases with the retarding of injection timing. UHC and CO emissions have relation to
the fuel spray penetration at different injection timings. NOx emissions increase rapidly with the increase
of the stratification degree.
Crown Copyright 2008 Published by Elsevier Ltd. All rights reserved.
1. Introduction
The homogenous charge compression ignition (HCCI) engine
has the potential to meet the increasingly stringent emission reg-
ulations. Pure HCCI combustion does not involve flame propaga-
tion or flame diffusion as in conventional internal combustion
engines. The main objective of HCCI combustion is to reduce soot
and NOx emissions while maintaining high fuel efficiency at part-
load conditions. However, several technical barriers must be over-
come before HCCI can be implemented in production engines.
Notably ways must be found to control ignition timing [1], expand
its limited operating range [2] and limit the rate of heat release [3].
Since the ignition delay is highly dependent on in-cylinder temper-
ature, pressure and fuelair ratio etc [4], cylinder-to-cylinder vari-
ations can also cause problems in HCCI engines [2]. Much of theprevious experimental work related to HCCI engine process has
been directed under conditions of homogeneous in-cylinder tem-
perature and composition, most commonly achieved using very
early premixing or port fuel injection strategies with careful tem-
perature management [5,6]. Solving the HCCI control problems
has led to the investigation of various control strategies that may
move away from truly homogeneous mixtures, including direct-
injection (DI) [7], i.e. charge stratification combustion.
Charge stratification combustion is a possible solution to the
control and specific power challenges of HCCI engines. With homo-
geneous charge, 10% of the fuel can exit in the unburned regions
[8] and this amount of the fuel does not contribute to the pressure
rise. If one imagines that all of the fuel was supplied via direct-
injection to the cylinder with less fuel in the quenching zones, it
would be possible to reduce the amount of the fuel residing in
the quenching zones in stratification combustion. Consequently,
the fuel economy could be improved and the HCCI operating range
can be expanded. Stratification combustion has been studied over
the past years by many researchers in engine experiments and
simulation studies [911]. Richter and coworkers [9] performed
engine imaging experiments to assess the magnitude and role of
inhomogeneities in HCCI operation. They concluded that charge
inhomogeneities were potentially significant and played an impor-tant role in the combustion process. Aceves and coworkers [10]
have considered the role of fuel structure and equivalence ratio
in extending combustion duration and controlling combustion
phasing. Iida and his colleague [11] investigated the influence of
the inhomogeneity in fuel distribution in the pre-mixture on the
HCCI combustion process experimentally by the chimilumines-
cence measurement. The results show that the use of varying the
inhomogeneity in fuel distribution in the pre-mixture is effective
as a method for controlling the combustion duration in HCCI
engines. The results of these researches show that it is possible
to influence and control the HCCI combustion by charge stratifica-
tion. Mixture stratification modifies local equivalence ratios and
0016-2361/$ - see front matter Crown Copyright 2008 Published by Elsevier Ltd. All rights reserved.doi:10.1016/j.fuel.2008.09.002
* Corresponding author. Tel.: +86 22 27406842x8014; fax: +86 22 27383362.
E-mail addresses: [email protected] (Z. Zheng), y_mingfa@tju.
edu.cn (M. Yao).
Fuel 88 (2009) 354365
Contents lists available at ScienceDirect
Fuel
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / f u e l
mailto:[email protected]:y_mingfa@tju.%20edu.cnmailto:y_mingfa@tju.%20edu.cnhttp://www.sciencedirect.com/science/journal/00162361http://www.elsevier.com/locate/fuelhttp://www.elsevier.com/locate/fuelhttp://www.sciencedirect.com/science/journal/00162361mailto:y_mingfa@tju.%20edu.cnmailto:y_mingfa@tju.%20edu.cnmailto:[email protected] -
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has been suggested as a potential mechanism for controlling HCCI
combustion. On the other hand, fuel stratification may produce
high NOx emissions. The challenge for stratification combustion
is to meet the control and high power output requirements of
modern engines while keeping NOx emissions low enough to meet
current and future emissions standards. Both the charge-stratified
combustion cited in the introduction and the stratification com-
bustion investigated in this study belong to general HCCI concept.They are compression ignition combustion that the pre-mixture is
inhomogeneous fueled with low octane number. The purpose of
these researches is to investigate the effects of inhomogeneity in
fuel distribution in the cylinder on combustion and emission.
With HCCI, the start of combustion is dominated by auto-igni-
tion chemical kinetics. Detailed chemical kinetics is usually used
to simulate HCCI combustion. Naik et al. [12] developed a surro-
gate gasoline reaction mechanism including five component fuels
ofiso-octane, n-heptane, 1-pentene, toluene, and methyl-cyclohex-
ane. The mechanism consists of 1328 species and 5835 reactions.
Predictions are in reasonably good agreement with the HCCI en-
gine data. Andrae et al. [13] developed a kinetic model for the
auto-ignition of toluene reference fuels (TRF) with two component
fuels of n-heptane and toluene. Good agreement between experi-
ments and predictions was found when the model was validated
against shock tube autoignition delay data for gasoline surrogate
fuels. In theory, any engine combustion problem could be solved
by linking a fluid mechanics code with a chemical kinetics code
including HCCI combustion and charge stratification combustion
where partial composition stratification exists. Multi-dimensional
computational fluid mechanics (CFD) models coupled directly with
chemical kinetics can analyze effects of the non-uniformities on
auto-ignition and combustion, and they can also analyze effects
of the in-cylinder turbulence on combustion. Daisho and his col-
league [14] developed a multi-dimensional model combined with
a detailed kinetics by the link between KIVA-3 and CHEMKIN-II
with some modifications to investigate the chemical reaction phe-
nomena encountered in the HCCI combustion process of natural
gas. Flowers et al. [15] used a parallelized fully coupled CFD andmulti-zone chemical kinetics solver (KIVA3V-MZ-MPI) to simulate
HCCI and stratification combustion. They imposed two stratifica-
tion cases to directly compare cases where the only difference is
the homogeneous and inhomogeneous nature of the fuelair
mixture.
The basic idea of stratification combustion is to enhance mixing
and evaporation by an early fuel injection in the compression cy-
cles. The injection system and the mixture formation are means
to control the emissions [16]. Though an early injection strategy
will enhance mixing, it is not applicable to all operating conditions.
If higher amounts of fuel are injected, the start of injection (SOI)
has to be advanced to earlier times in order to separate the ignition
and evaporation process. Therefore, fuel-wetting on the liner due
to spraywall interactions leads to high unburned hydrocarbon(UHC) emissions. For this work, port injection was used for the
main fuel supply to create a homogenous air fuel mixture. In order
to obtain a stratified charge, the other fuel was delivered by spray-
guided direct-injection. By altering the ratio of direct-injected fuel
to the total fuel and by retarding the injection timing of direct-in-
jected fuel, the fuel-wetting can be avoided successfully and
degree of charge stratification can also be controlled. n-Heptane
(n-C7H16) is selected as fuel, which has a cetane index of 56, equiv-
alent to that of a diesel fuel. A fully coupled multi-dimensional CFD
code (Star/Kinetics) and chemical kinetics code was used to inves-
tigate charge stratification combustion. For n-heptane, detailed
chemical kinetic (thousands of reactions and hundreds of species)
calculations coupled with CFD simulations of chemically reacting
flows are still unrealistic as the basis for a parametric simulationtool due to taking large amounts of CPU time. Therefore, reduced
mechanism is adopted. The analysis focuses on how stratification
of pre-mixture affects ignition, combustion and emissions.
2. Experiment and computational model
2.1. Engine experiments for model validation
The experiment was conducted on single-cylinder engine whichis changed from a four-cylinder, water-cooled, four-stroke, and di-
rect-injection diesel engine. As the cylinder for experiment, the
first cylinder has independent intake and exhaust systems which
are separate from the other three cylinders. To achieve port-injec-
tion/direct-injection mode, two independent electronic-controlled
fuel supply systems were used. Engine specifications are given in
Table 1.
Stratification of the charge was varied in two ways: (1) by
retarding the injection timing of direct-injection; (2) by altering
the ratio of the direct-injected fuel to the total fuel supplied to
the system. One measure of stratification is defined by the DI ratio.
The DI ratio is defined as the ratio of the direct-injected fuel mass
to the total fuel mass supplied to the system.
2.2. The improvements and validation of the reduced model
The reduced n-heptane mechanism [17] which adapts to HCCI
combustion (very lean mixture) in our earlier work has been devel-
oped from the Lawrence Livermore National Laboratory (LLNL) de-
tailed n-heptane mechanism which includes 544 chemical species
and 2446 elementary reactions [18]. In stratification combustion,
stratification leads to the existence of local high fuel concentration
regions. Therefore, the reduced mechanism must be extended to
meet wider fuel concentration range and this work is also based
on the LLNL n-heptane mechanism. The computational conditions
of the following analysis are the same as the conditions in the
developing of the original HCCI reduced mechanism except the
equivalence ratio (naturally aspirated HCCI engine; compression
ratio of 17; initial temperature of 350 K and engine speed of1400 r/min). Fig. 1 shows sensitivity analysis (mass fraction) of
the detailed mechanism at the equivalence ratio of 1.5. The sensi-
tivity coefficients of reactions is collected at 10.32oATDC when
maximal coefficients appears in the high temperature reaction
stage. Remarkably, in the original reduced mechanism for HCCI
combustion (very lean mixture), the three reactions in Fig. 1a were
not included because the sensitivity coefficients of the three reac-
tions were so small that they did not appear in the sensitivity anal-
ysis. Fig. 1a indicates that the three reactions become more and
more important with the increase of the equivalence ratio. There-
fore, reactions C7H13 = C3H5-A + C4H8-1, C7H14-2 + OH = C7H13 +
H2O and C4H8-1 = C3H5-A + CH3 are added to the new reduced
mechanism.
Obviously, the three reactions introduce C7H14-2 and C3H5-A tothe new reduced mechanism. Therefore, the generation path of
Table 1
Engine specifications
Bore 112 mm
Stroke 132 mm
Displacement 1300 cm3
Compression ratio 17.5:1
Engine speed 1400 r/min
Intake valve open 13.5oBTDC
Intake valve close 38.5oABDC
Exhaust valve open 56.5oBBDC
Exhaust valve close 11.5oATDC
Swirl ratio 2.0
Z. Zheng, M. Yao/ Fuel 88 (2009) 354365 355
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C7H14-2 and consumption path of C3H5-A should be included in the
new reduced mechanism.
In the original reduced mechanism, C7H15-3 was selected as the
product of H-atom abstraction among four distinct heptyl radicals
of the detailed mechanism. Furthermore, there is only one reaction
path from C7H15-3 to C7H14-2: C7H15-3 + O2 = C7H14-2 + HO2.
When the mixture is very rich, this reaction competes with
the first O2 addition to heptyl radicals, which inhabits the low
temperature branching. Fig. 2 shows the in-cylinder pressure and
heat release rate at the equivalence ratio of 1.5. It can be seen that
the low temperature reaction is not obvious when the mixture is
very rich.
If the fuel and air cannot mix with each other perfectly
before ignition, the local high fuel concentration regions may
probably lead to the increase of soot emissions. It is well known
that C2H2 is precursor of PAHs (Polycyclic Aromatic Hydrocarbons)
and soot, Therefore, C2H2 is introduced into the new reduced
mechanism for stratification combustion by following reactions
which indicate the consumption path of C3H5-A: C3H5-A =
C2H2 + CH3; C2H2 + OH = CH2CO + H.
-0.03
-0.02
-0.01
0.00
0.01
0.02
C7H
13= C
4H
8-1+C
3H
5-A C
4H
8-1= C
3H
5-A+CH
3
C7H
14-2+OH = C
7H
13+H
2O
Normalized
Sensitivities
(mass
fraction)
-0.07
-0.06
-0.05
-0.04
-0.03
-0.02
-0.01
0.00
C2H
5+HO
2= C
2H
5O+OH
CH3+HO
2= CH
3O+OHN
ormalizedSensitivities
(mass
fraction)
(a) Sensitivity coefficients of three reactions not existing inoriginal reduced mechanism
(b) Sensitivity coefficients of the reactions of CH3 and C2H5with HO2
Fig. 1. Sensitivity analysis (mass fraction) of the detailed mechanism at the equivalence ratio of 1.5.
-25 -20 -15 -10 -5 0 5 10 15 20 25-2
0
2
4
6
810
12
14
16
18
In-cylinderPres
sure/MPa
Crank Angle /oCA
= 1.5n=1400r/min
-25 -20 -15 -10 -5 0 5 10 15 20 25
0
1000
2000
3000
4000
5000
6000
7000
8000
n=1400r/min
RateofHeatRelease/J.(oCA)-1
Crank Angle /oCA
= 1.5
Fig. 2. In-cylinder pressure and heat release rate at the equivalence ratio of 1.5.
-25 -20 -15 -10 -5 0 5 10 15 20 25
-0.05
0.00
0.05
0.10
0.15
C2H
4+OH = CH
3+CH
2O
C2H
4+OH = C
2H
3+H
2O
C2H
5+O
2= C
2H
4+HO
2
ReactionRate/mol.(g.s)-1
ReactionRate/mol.(g.s)-1
Crank Angle /oCA ATDC
-25 -20 -15 -10 -5 0 5 10 15 20 25
Crank Angle /oCA ATDC
-0.08
-0.06
-0.04
-0.02
0.00
0.02
0.04
0.06
C2H
4+OH = CH
3+CH
2O
C2H
4+OH = C
2H
3+H
2O
C2H
5+O
2= C
2H
4+HO
2
(a) At equivalence ratio of 1.5 (b) At equivalence ratio of 0.264
Fig. 3. Reaction rates of C2H4 at the equivalence ratios of 1.5 and 0.264.
356 Z. Zheng, M. Yao / Fuel 88 (2009) 354365
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Fig. 1b shows the sensitivity coefficients of the reactions of CH3and C2H5 with HO2 at the equivalence ratio of 1.5. It can be seen
that the contribution of reaction CH3 + HO2 = CH3O + OH to the
whole reaction system is more than reaction C2H5 + HO2= C2H5O + OH. Moreover, more generation paths of CH3 are in-
cluded in the new reduced mechanism compared with C2H5.
Therefore, reaction CH3 + HO2 = CH3O + OH is added to the new re-
duced mechanism, while reaction C2H5 + HO2 = C2H5O + OH and itsrelevant reaction C2H5O = CH3 + CH2O are removed. In addition, an-
other consumption reaction of CH3 and its relevant reaction,
CH3 + OH = CH2O + H2 and OH + H2 = H + H2O, are added too.
Fig. 3a and b shows the reaction rates of C2H4 at the equivalence
ratios of 1.5 and 0.264, respectively. At the equivalence ratio of
0.264 (Fig. 3b), the reaction rate of one consumption reaction,
C2H4 + OH = CH3 + CH2O, is much smaller than the reaction rate
of the other reaction C2H4 + OH = C2H3 + H2O. Consequently, reac-
tion C2H4 + OH = C2H3 + H2O was included in the original reducedmechanism while reaction C2H4 + OH = CH3 + CH2O was not in-
Table 2
Species and reactions of new n-heptane reduced mechanism for stratification combustion
Reaction considered (k = AT**n exp(E/RT))
A+ A n+ n E+ E
Reactions included in
the old reduced
mechanism of HCCI
combustion
1. NC7H16 + OH = C7H15-3 + H2O 9.40E+07 4.60E+09 1.6 1.3 40 22,220
2. NC7H16 + HO2 = C7H15-3 + H2O2 4.88E+12 1.94E+12 0.0 2.3 7409 35,750
3. NC7H16 + O2 = C7H15-3 + HO2 4.00E+13 4.07E+12 0.0 0.0 47,600 0.0
4. C7H15O2-3 = C7H15-3 + O2 2.97E+22 2.00E+12 2.3 0.0 35,750 0.0
5. C7H15O2-3 = C7H14OOH3-5 4.96E+11 2.80E+10 0.0 0.1 22,150 11,830
6. C7H14OOH3-5O2 = C7H14OOH3-5 + O2 7.95E+22 2.00E+12 2.5 0.0 35,820 0.0
7. C7H14OOH3-5O2 = NC7KET35 + OH 1.24E+13 3.14E+03 0.0 1.8 19,150 46,3808. NC7KET35 = C2H5CHO + C2H5COCH2 + OH 3.98E+15 0.00E+00 0.0 0.0 43,000 0.0
9. C7H14OOH3-5 = OH + C2H5CHO + C4H8-1 5.00E+13 0.00E+00 0.0 0.0 25,500 0.0
10. C7H15-3 = C4H8-1 + NC3H7 2.80E+11 8.50E+10 0.2 0.0 23,010 7800
11. H2O2 + OH = H2O + HO2 2.40E+00 4.04E01 4.0 4.4 2162 29,300
12. H2O2 + O2 = HO2 + HO2 5.94E+17 4.20E+14 0.7 0.0 53,150 11,980
13. OH + OH(+M) = H2O2(+M) 1.24E+14 0.00E+00 0.4 0.0 0.0 0.0
14. H2O2 + O2 = HO2 + HO2 1.84E+14 1.30E+11 0.7 0.0 39,550 1629
15. H + O2 = O + OH 1.92E+14 1.52E+13 0.0 0.0 16,440 325.0
16. O + H2O = OH + OH 1.21E+05 1.23E+04 2.6 2.6 15,370 1878
17. C2H3 + O2 = CH2O + HCO 4.00E+12 4.00E+12 0.0 0.0 250 86,300
18. CH2O + OH = HCO + H 2O 3.43E+09 2.35E+08 1.2 1.4 447 26,120
19. HCO + M = H + CO + M 1.86E+17 6.47E+13 1.0 0.0 17,000 442
20. HCO + O2 = CO + HO2 9.10E+12 1.69E+14 0.0 0.3 410 34,590
21. CO + OH = CO2 + H 9.43E+03 1.06E+06 2.3 2.3 2351 19,980
22. C2H5 + O2 = C2H4 + HO2 1.22E+30 1.26E+30 5.8 5.6 10,100 22,310
23. C2H4 + OH = C2H3 + H2O 2.02E+13 1.02E+13 0.0 0.0 5955 20,220
24. C2H5CO = C2H5 + CO 1.00E+11 3.00E+09 0.0 1.0 10,000 747125. C2H5CHO + OH = C2H5CO + H2O 1.00E+13 1.91E+13 0.0 0.0 2000 36,620
26. C2H5COCH2 = CH2CO + C2H5 1.57E+13 2.11E+11 0.0 0.0 30,000 7350
27. C4H8-1 + OH = C4H7 + H2O 2.25E+13 4.77E+12 0.0 0.0 2217 26,470
28. C4H7 + O2 = C4H6 + HO2 1.00E+09 1.00E+11 0.0 0.0 0.0 17,000
29. C4H7 + HO2 = C4H7O + OH 1.900E+12 2.000E+10 0.0 0.0 1200 0.0
30. C4H6 + OH = C2H5 + CH2CO 1.00E+12 3.73E+12 0.0 0.0 0.0 30,020
31. CH2CO + OH = CH2O + HCO 2.80E+13 2.76E+13 0.0 0.0 0.0 18,500
32. C4H7O = CH3CHO + C2H3 7.94E+14 1.00E+10 0.0 0.0 19,000 20,000
33. CH3CHO + OH = CH3CO + H2O 1.00E+13 1.90E+13 0.0 0.0 0.0 36,620
34. CH3CO + M = CH3 + CO + M 1.00E+12 3.73E+12 0.0 0.0 0.0 30,020
35. HO2 + M = H + O2 + M 6.85E+19 2.00E+17 1.5 0.8 49,960 0.0
36. CH3 + O2 = CH3O + O 4.80E+13 3.04E+14 0.0 0.0 29,000 733
37. CH3O + O2 = CH2O + HO2 7.60E+10 1.28E+11 0.0 0.0 2700 32,170
38. CH2O + HO2 = HCO + H2O2 5.60E+12 7.79E+11 0.0 0.0 13,600 10,230
39. NC3H7 = CH3 + C2H4 9.47E+13 1.70E+11 0.6 0.0 29,000 7800
Reactions added to the
new reduced mechanism
40. C7H15-3 + O2 = C7H14-2 + HO2 3.00E09 3.79E09 0.00 0.05 3000 18,270
41. C7H14-2 + OH = C7H13 + H2O 3.00E+13 1.60E+15 0.00 0.63 1230 33,610
42. C7H13 = C3H5-A + C4H8-1 2.50E+13 1.00E+13 0.00 0.00 45,000 9600
43. C4H8-1 = C3H5-A + CH3 1.50E+19 1.35E+13 1.00 0.00 73,400 0.0
44. C3H5-A = C2H2 + CH3 4.46E+46 2.61E+46 9.49 9.82 81,290 36,950
45. C2H2 + OH = CH2CO + H 3.20E+11 3.16E+12 0.00 0.00 200 20,860
46. CH3 + HO2 = CH3O + OH 1.99E+13 8.65E+14 0.00 0.35 0.0 24,550
47. CH3 + OH = CH2O + H2 4.00E+12 1.20E+14 0.00 0.00 0.0 71,720
48. OH + H2 = H + H2O 2.16E+08 9.35E+08 1.51 1.51 3430 18,580
49. C2H4 + OH = CH3 + CH2O 2.00E+12 6.00E+11 0.00 0.00 956 16,480
50. N + CO2 = NO + CO 1.90E+11 0.00 3400
51. N2O + O = N2 + O2 1.40E+12 0.00 10,810
52. N2O + O = NO + NO 2.90E+13 1.00 23,150
53. N2O + H = N2 + OH 4.40E+14 0.00 18,880
54. N2O + OH = N2 + HO2 2.00E+12 0.00 21,060
55. N2O + M = N2 + O + M 1.30E+11 0.00 59,620
56. N + NO = N2 + O 3.27E+12 0.30 0.0
57. N + O2 = NO + O 6.40E+09 1.00 6280
58. N + OH = NO + H 7.33E+13 0.00 1120
K, rate constant; A, pre-exponential factor; n, temperature exponent; E, activation energy (+, forward direction, , reverse direction).
Z. Zheng, M. Yao/ Fuel 88 (2009) 354365 357
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cluded in it. At the equivalence ratio of 1.5 (Fig. 3a), the situation is
different and the reaction rates of above two reactions are similar,
that is to say, they are both main consumption reactions of C 2H4 at
the equivalence ratio of 1.5. Therefore, reaction C2H4 + OH = CH3 + -CH2O is added to the new reduced mechanism.
In addition, NOx model which includes nine reactions is ob-
tained from the n-heptane reaction mechanism of Golovitchev
[19]. The new reduced mechanism includes 42 species and 58 ele-
mentary reactions. All species and reactions are shown in Table 2.
Fig. 4 shows the comparison of pressure profiles calculated from
the detailed mechanism and the new reduced mechanism respec-
tively at four equivalence ratios of 1.5, 0.5, 0.264 and 0.2. The initial
pressure and temperature are the same as Figs. 13 and there is no
charge stratification. Fig. 4 indicates that the predictions of the
new reduced mechanism and the detailed mechanism are in excel-
lent agreement within wide equivalence ratio range.
2.3. Fully coupled CFD and chemical kinetics model
The new n-heptane reduced mechanism for stratification com-
bustion was used to simulate the fuel chemistry in this study. It
has been implemented in the STAR/KINetics CFD code to simulate
the combustion with an inhomogeneous charge. The KINetics mod-
ule incorporates CHEMKIN technology for formulating heteroge-
neous and gas-phase chemistry with an advanced solver
approach specifically designed to work with the CFD software,
STAR-CD [20]. The STAR-CD code provides CHEMKIN the species
and thermodynamic information of the computational cells and
the CHEMKIN code returns the new species information after solv-
ing the chemistry. The chemistry and flow solutions are then
coupled.
The RNGje
model was used for turbulence modeling. The PISOalgorithm was used for the transient flow of the engine. At each
cell, the complex chemical kinetics during the HCCI combustion
is dealt with the built-in CHEMKIN module. After the solutions
for all cells, the mass transfer, heat transfer between cells and
the flow are simulated by the corresponding sub-models. Thenthe interaction between turbulent mixing and chemical reaction
are implemented. The injection process being modeled includes
the flow in the nozzle hole and atomization process. In this study,
these properties are calculated on the basis of Huh atomization
model, ReitzDiwakar breakup model. In addition, Bai spray
impingement model is adopted to describe the processes of drop
rebound, spread and splash.
Kong and Reitz [21] investigated sensitivity to grid density in
premixed HCCI engine. The results indicate that premixed HCCI
combustion simulations can be achieved by using coarse mesh
CFD with detailed chemistry. However, because of spray simula-
tions in this study, the results will be relatively sensitive to the res-
olution of the numerical grid. The numerical solution of the
NavierStokes equations becomes more and more accurate if thegrid is refined. Abraham [22] has shown that the jet cross-sectional
area has to be resolved by at least four grid cells near the nozzle in
the case of a gas jet being injected into a gas atmosphere. On the
other hand, because it is impractical to follow each individual drop
inside a spray, the combination of MonteCarlo method and sto-
chastic parcel technique is used in order to reduce the number of
individual drops the behavior of which has to be directly calcu-
lated. The more parcels are used, the better the behavior of the dis-
persed liquid phase is resolved, and the better the statistical
convergence. If the grid size is too small, the parcels in the cell
are not enough to ensure numerical stabilities. Therefore, appropri-
ate grid size should meet numerical accuracies and stabilities.
Based on the results of a mass of computation, it is summarized
that the grid size is between 1 and 2 mm and the time step is0.1oCA can obtain good numerical accuracies and stabilities in
-60 -40 -20 0 20 40 60
-60 -40 -20 0 20 40 60 -60 -40 -20 0 20 40 60
-60 -40 -20 0 20 40 60
0
2
4
68
10
12
14
16
18
20
Crank Angle /oCAATDC Crank Angle /
oCAATDC
Crank Angle /oCAATDC Crank Angle /
oCAATDC
Detailed mechanism [18]
Reduced mechanism
In-cylinderPressure/MPa
=1.5n=1400r/min
0
2
4
6
8
10
12
14
In-cylinder
Pressure/MPa
= 0.5n=1400r/min
Detailed mechanism [18]
Reduced mechanism
0
2
4
6
8
10Detailed mechanism [18]
Reduced mechanism
In-cylinderPr
essure/MPa
= 0.264n=1400r/min
0
2
4
6
8
10Detailed mechanism [18]
Reduced mechanism
In-cylinderPressure/MPa
= 0.2n=1400r/min
Fig. 4. Comparison of pressure profiles between the detailed mechanism and the new reduced mechanism.
358 Z. Zheng, M. Yao / Fuel 88 (2009) 354365
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the condition of bore diameter and engine speed of current diesel
engine. Furthermore, the computational time can be acceptable.
The meshes of combustion chamber were shown in Fig. 5a. The
injector contains seven holes with identical diameter of
0.164 mm. To reduce the computation time, only a sector of 51o
was used in the simulation with single injection of the injector.
The multi-dimensional computations started at intake valve close
(IVC) and ended at exhaust valve open (EVO).
2.4. Comparisons of pressure profiles between experiments and
calculation
Fig. 5b shows the comparison of cylinder pressure and NOxemissions between simulation and experiment. It can be seen that
the computed results agree well with the measured results in both
in-cylinder profiles and emission trends.
3. Results and discussion
Fig. 5c shows predicted mass history profiles of fuel and impor-
tant intermediate species of stratification combustion at the condi-
tion of a DI ratio of 15% and an injection timing of43ATDC. 15%
direct-injected fuel leads to the increase of fuel mass from
43ATDC. After atomization and evaporation, this part of fuel
mixes with homogeneous mixture of fuel and air in the cylinder
and the mass of total fuel keeps constant before the low tempera-
ture reaction occurs. Similar to HCCI combustion, the low temper-ature reaction of the combustion of the premixed/direct-injected
fuel starts about at 24ATDC. Most fuel is consumed at this stage.
Some important intermediate species such as CH2O and H2O2 are
rapidly formed as soon as the low temperature reaction occurs.
When the high temperature reaction begins, these two species
are quickly consumed. Moreover, the mass of CO gets to the peak
value as CH2O and H2O2 masses relative to their minimum value.
Fig. 6 shows the predicted distributions of in-cylinder tempera-
ture and fuel concentration. There is some variation along the rota-
tional angle keeping r and z constant because the local high fuel
concentration regions are formed by atomization and evaporation.
The slice crossed the section of spray axis is selected in this study
to clearly observe the local high fuel concentration and tempera-
ture regions inside the combustion chamber. Direct-injected fuelleads to local high fuel concentration regions in the cylinder. Be-
cause atomization and evaporation of direct-injected fuel are
endothermic processes, the local in-cylinder temperatures in the
regions involved in the spray process are lower than those of other
regions with homogeneous fuel at 24ATDC. Therefore, the low
temperature reaction occurs in the regions with homogeneous fuel
first and homogeneous fuel in these regions starts to be consumed
at this time. At the end of the low temperature reaction
(20ATDC), the in-cylinder temperature is almost uniform except
strong heat transfer regions such as the piston-ring crevice regions
and the regions near the piston surface. Most fuel is consumed at
20ATDC and relatively local high fuel concentration regions are
still in existence. With the piston going up, it can be seen from
Fig. 6a that the temperature regions above 1000 K appears in highfuel concentration regions first at 13ATDC. This indicates that
-45 -40 -35 -30 -25 -20 -15 -10 -5 0 50.0
5.0x10-7
1.0x10-6
1.5x10
-6
2.0x10-6
2.5x10-6
3.0x10-6
3.5x10-6
0.0
1.0x10-7
2.0x10-7
3.0x10-7
4.0x10-7
5.0x10-7
CH2
Oa
ndH2
O2
/kg
Fuela
ndCO/kg
Crank Angle /oCA
Fuel
CO
H2O
2
CH2O
DI ratio = 15%
SOI = -43
o
ATDC
(a) Engine combustion chamber geometry and computationalmesh
(c) Predicted mass history profiles of fuel and importantintermediate species
-80 -60 -40 -20 0 20 40 60 800
2
4
6
8
10Measured
Calculated
In-cylinderPressure
/MPa
Crank Angle /oCA
n=1400 r/min
=0.3DI ratio=20%
SOI=-25 deg.ATDC
20% 30% 40%
60
80
100
120
140
160
180
200
220
240
260
280
300n=1400 r/min
=0.3SOI=-60 deg.ATDC
NOx/ppm
DI Ratio
Measured
Calculation
(b) Comparison of cylinder pressure and NOx emissions between simulation and experiment
Fig. 5. Computational mesh, model validation and mass history profiles of species.
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high temperature reaction begins from these high fuel concentra-
tion regions. As soon as the high temperature begins, residual fuel
continues to be oxidized (Figs. 5c and 6b) and CH2O and H2O2 are
rapidly consumed (Fig. 5c). The high temperature reaction ends at
6ATDC. As shown in Fig. 6, the fuel is completely consumed ex-
cept the piston-ring crevice regions at 6ATDC. As to in-cylinder
temperature, local high temperature regions appear in high fuelconcentration regions due to direct-injection. The increase of NOxemissions attributes to these local high temperature regions in
stratification combustion.
3.1. Effects of DI ratio on combustion and emissions
Fig. 7a and b shows the predicted effects of DI ratio on the in-
cylinder pressure and emissions. The dashed line in Fig. 7a indi-
cates the predicted in-cylinder pressure profile of HCCI combustion
at the same operating conditions. Compared with HCCI combus-
tion, there are no obvious effects on the in-cylinder pressure when
the DI ratio is 5%. The reason is that 5% direct-injected fuel can only
lead to slight stratification which has no obvious effects on the
pressure profiles. For the present three DI ratios, the onsets ofthe low temperature reaction of stratification combustion are the
same as HCCI combustion. However, the effects of stratification
on the high temperature reaction are more and more obvious with
the increase of DI ratio. With the increase of DI ratio, the onset of
the high temperature reaction advances, furthermore, the maxi-
mum pressure rise rate decreases. This is benefit to prolonging
the combustion duration and expanding of the operation range
to higher loads.
Fig. 8 shows predicted equivalence ratio and in-cylinder tem-
perature distributions at different timings for three DI ratios with
the injection timing of33ATDC. It can be seen from Fig. 8a that
the local high equivalence ratio increases with the increase of DI
ratio. That is to say, the stratification degree of the mixture in-
creases with the increase of DI ratio. Combined with Fig. 8b whichpresents the in-cylinder temperature distributions when the high
temperature reaction just occurs, it can be concluded: with the in-
crease of DI ratio, the local high equivalence ratio increases, while
the fuel in the homogeneous mixture decreases. Therefore, the big-
ger the DI ratio is, the earlier the local high temperature regions
meeting ignition condition appear and the lower the temperatures
in the strong heat transfer regions such as the regions of cylinder
wall and piston surface are. The local high temperature regionsthat appear earlier advance the ignition timing. In addition, other
lower temperature regions which are much larger than the local
high temperature regions lead to the decrease of the combustion
rate and the reduction of the pressure rise rate.
Fig. 7b shows the predicted effects of DI ratio on UHC, CO and
NOx emissions (the injection timing of33ATDC as an example).
All the cases at three DI ratios and three injection timings in this
study do not lead to wall-wetting, so the direct-injected fuel can-
not enter into the piston-ring crevice regions with the piston going
up. It is well known that UHC emissions are mainly from the pis-
ton-ring crevice regions for HCCI combustion. Therefore, compared
with HCCI combustion, UHC emissions decrease because the fuel in
the piston-ring crevice regions becomes less than that of HCCI
combustion. Furthermore, the fuel in the piston-ring crevice re-gions decreases with the increase of the DI ratio. This results in
the decrease of UHC emissions with the increase of DI ratio. CO
emissions at all DI ratios are higher than that of HCCI combustion
and CO emissions increase with the increase of DI ratio. It can be
seen from Fig. 8b that the temperature decreases in the strong heat
transfer regions such as the regions of cylinder wall and piston sur-
face with the increase of DI ratio and the low temperature regions
are extended. This leads to stronger flame quenching in the strong
heat transfer regions for stratification combustion and more CO
cannot be oxidized to CO2. NOx emissions increase with the in-
crease of DI ratio. NOx formation in engines occurs primarily
through the high temperature Zeldovich NOx formation mecha-
nism [23], which does not produce significant NOx until tempera-
ture exceed$
1900 K. At temperature higher than 1900 K,Zeldovich mechanism results in an exponential increase in NOx
Fig. 6. Predicted in-cylinder temperature and fuel concentration distributions at different crank angles.
360 Z. Zheng, M. Yao / Fuel 88 (2009) 354365
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formation. Fig. 8c gives the predicted in-cylinder temperature dis-
tributions at the end of the high temperature reaction at three DI
ratios. When DI ratio is 15%, the maximum in-cylinder tempera-
ture is about 1900 K; if the DI ratio continues to increase, NOxemissions increase more rapidly. When DI ratio is 25%, the maxi-
mum in-cylinder temperature has exceeded 2200 K. Therefore,
NOx emissions rise dramatically from very low values and increase
by at least two orders of magnitude to values typical of current EGR
controlled diesel engines. Though current EGR controlled diesel en-
gines can reduce NOx emissions, soot emissions increase at the
same time because of the trade-off relationship between NOx and
soot in convention diesel engine. In this study, the combustion of
all stratification cases is premixed combustion. Therefore, sootemissions are much lower than conventional combustion con-
trolled with EGR. Consequently, there is definite meaning investi-
gating this kind of HCCI combustion though NOx emissions
increase.
3.2. Effects of injection timing on combustion and emissions
Fig. 9a shows the predicted effects of injection timing on in-cyl-
inder pressure at each DI ratio of this study. The dashed line in
Fig. 9a indicates the predicted in-cylinder pressure profile of HCCI
combustion at the same operating conditions. When DI ratio is 5%,
all the in-cylinder pressure profiles at three injection timings are
similar to that of HCCI combustion. This is because the direct-in-
jected fuel is too little to have obvious effects on the in-cylinderpressure at any injection timing of this study. When the DI ratio
-25 -20 -15 -10 -5 0 5 100
1
2
3
4
5
6
7
8
9SOI = -33
oATDC
DI ratio = 5%
DI ratio = 15%
HCCI
In-cylind
erPressure/MPa
Crank Angle /oCA
DI ratio = 25%
-25 -20 -15 -10 -5 0 5 100
1
2
3
4
5
6
7
8
9SOI = -43
oATDC
DI ratio = 5%
DI ratio = 15%
DI ratio = 25%
HCCI
In-cylind
erPressure/MPa
Crank Angle /oCA
-25 -20 -15 -10 -5 0 5 10-1
0
1
2
3
4
5
6
7
8
9
HCCI
DI ratio = 5%
DI ratio = 15%
DI ratio = 25%
SOI = -50oATDC
In-cylinderPressure/MPa
Crank Angle /oCA
(a) Predicted effects of DI ratio on the in-cylinder pressure
0.00 0.05 0.10 0.15 0.20 0.251.45
1.50
1.55
1.60
1.65
1.70
1.75
1.80
1.85
1.90
1.8
1.9
2.0
2.1
2.2
2.3
FuelCarbonintoCO
/%
FuelCarbonintoUHC
/%
HC
CO
5% 15% 25%0.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4 NOx
NOX
(g/KW.h)
DI RatioDI Ratio
(b) Predicted effects of DI ratio on UHC, CO and NOx emissions (SOI=-33ATDC)
Fig. 7. Predicted effects of DI ratio on the in-cylinder pressure and emissions.
Z. Zheng, M. Yao/ Fuel 88 (2009) 354365 361
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is 15%, compared with HCCI combustion, the high temperature
ignition timings of three stratification combustion cases advance
slightly and different injection timings have a few effects on the
ignition timing and pressure rise rate. When the DI ratio is 25%,
compared with HCCI combustion, the high temperature ignition
timings of three stratification combustion cases significantly ad-
vance. Furthermore, with the retarding of the injection timing,
the high temperature ignition timing advances and the pressurerise rate decreases. From above analysis, it can be concluded that
the effects of injection timing are less than those of the DI ratio
in this study and have close relation to the DI ratio: The effects
of injection timing increase with the increase of DI ratio. Fig. 10
presents the predicted effects of injection timing on the equiva-
lence ratio distributions before the low temperature reaction oc-
curs. Compared with Fig. 10a and b, it can be seen that the
stratification degree differences among different injection timings
enlarge with the increase of DI ratio. Therefore, the effects of injec-
tion timing increase with the increase of DI ratio. It can also be
seen from Fig. 10b that when the DI ratio is 25%, the stratification
degree of the mixture increases significantly with the retarding of
the injection timing. Similar to the effects of DI ratio analyzed in
above section, the local high temperature regions in the local highfuel concentration regions that appear earlier advance the ignition
timing. In addition, other lower temperature regions which are
much larger than local high temperature regions lead to the de-
crease of the combustion rate and the reduction of the pressure
rise rate.
Fig. 9b shows the predicted effects of injection timing on UHC,
CO and NOx emissions at three DI ratios. When DI ratio is 5%,
retarding the injection timing has no obvious effects on three kinds
of emissions. 5% direct-injected fuel can only lead to slight stratifi-cation. Therefore, the effects of injection timing are inconspicuous
on both pressure profiles and emissions. However, compared with
HCCI combustion, UHC emissions decrease because the fuel in the
piston-ring crevice regions decreases; CO emissions increase be-
cause the leaner mixture in the strong heat transfer regions leads
to lower temperature there. When DI ratio is 15%, retarding the
injection timing still has little effects on UHC emissions. Until the
injection timing retards to 33ATDC, the stratification degree
leads to higher temperature to oxidize slight more UHC. Compared
with the two cases of the injection timings of 50ATDC and
43ATDC, the difference of temperature by different stratification
degree cannot oxidize more UHC. Therefore, UHC emissions are
similar at the two cases. However, the temperature difference
has resulted in slight more oxidization of CO. At the injection tim-ing of33ATDC, CO emissions continue to decrease slightly due to
Fig. 8. Predicted equivalence ratio and In-cylinder temperature distributions at different timings for three DI ratios (SOI = 33ATDC).
362 Z. Zheng, M. Yao / Fuel 88 (2009) 354365
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-25 -20 -15 -10 -5 0 5 100
1
2
3
4
5
6
7
8
9
-33,-43,-50oATDC
HCCI
In-cylinde
rPressure/MPa
Crank Angle /oCA
DI ratio = 5%
-25 -20 -15 -10 -5 0 5 10-1
0
1
2
3
4
5
6
7
8
9
-50oATDC
-43oATDC
-33oATDC
HCCIDI ratio = 15%
In-cylinderPressure/MPa
Crank Angle /oCA
-25 -20 -15 -10 -5 0 5 100
1
2
3
4
5
6
7
8
9
-33oATDC
-43oATDC
-50oATDC
HCCIDI ratio = 25%
In-cylinde
rPressure/MPa
Crank Angle /oCA
(a) Predicted effects of injection timing on in-cylinder pressure
-52 -50 -48 -46 -44 -42 -40 -38 -36 -34 -32
1.50
1.55
1.60
1.65
1.70
1.75
1.80
1.85
UHCHCCI
FuelCarboninto
UHC%
DI ratio = 25%
DI ratio = 15%
DI ratio = 5%
-52 -50 -48 -46 -44 -42 -40 -38 -36 -34 -321.8
1.9
2.0
2.1
2.2
2.3
2.4
2.5
CO
DI ratio = 25%
DI ratio = 15%
FuelCarboninto
CO%
DI ratio = 5%
HCCI
-52 -50 -48 -46 -44 -42 -40 -38 -36 -34 -320.0
0.2
0.4
0.6
0.8
1.0
1.2
1.4
DI ratio = 25%
DI ratio = 15%
DI ratio = 5%
NOx
NOX
(g/KW.h)
Injection Timing /oCA ATDC
Injection Timing /oCA ATDC Injection Timing /
oCA ATDC
HCCI
(b) Predicted effects of injection timing on UHC, CO and NOx emissions
Fig. 9. Predicted effects of injection timing on in-cylinder pressure and emissions at three DI ratios.
Z. Zheng, M. Yao/ Fuel 88 (2009) 354365 363
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4. Retarding injection timing can lead to the increase of stratifica-
tion in the condition of present premixed/direct-injected fuel
combustion. The effects of injection timing on combustion
and emissions have close relation to the DI ratio. Small DI ratio
has no obvious effects on the in-cylinder pressure and emis-
sions; at larger DI ratio, the onset of the high temperature reac-
tion advances and the maximum pressure rise rate decreases.
UHC and CO emissions have relation to the fuel spraypenetration.
5. NOx emissions increase with the increase of DI ratio and the
retarding of the injection timing in the condition of present pre-
mixed/direct-injected fuel combustion. NOx emissions increase
with the stratification degree.
Acknowledgements
The research is supported by the National Natural Science
Found of China (NSFC) through its project (50676066) and the
National Natural Science Found of China (NSFC) through its key
project Some key questions in advanced combustion and control
in engines (50636040).
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