Charge Stratification to Control HCCI

8/9/2019 Charge Stratification to Control HCCI

1/12

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]


2/12

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


3/12

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


4/12

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


5/12

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


= 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


6/12

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.

Z. Zheng, M. Yao/ Fuel 88 (2009) 354365 359


7/12

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


8/12

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


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


9/12

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


10/12

-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%


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


11/12


12/12

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).

References

[1] Wagner U, Anca R, Velji A, Spicher U. An Experimental Study of HomogenousCharge Compression Ignition (HCCI) with Various Compression Ratios, IntakeAir Temperatures and Fuels with Port and Direct Fuel Injection. SAE Paper,2003-01-2293.

[2] L XC, Chen W, Huang Z. A fundamental study on the control of the HCCIcombustion and emissions by fuel design concept combined with controllableEGR. Part 1. The basic characteristics of HCCI combustion. Fuel2005;84:107483.

[3] L XC, Chen W, Huang Z. A fundamental study on the control of the HCCIcombustion and emissions by fuel design concept combined with controllableEGR. Part 2. Effect of operating conditions and EGR on HCCI combustion. Fuel2005;84:108492.

[4] Glassman I. Combustion. 3rd ed. United States of America: Academic Press;1996.

[5] Yao MF, Zheng ZQ, Qin J. Experimental study on HCCI combustion with fuel ofdimethyl ether and natural gas. J Eng Gas Turbines Power 2006;128:41420.

[6] Yao MF, Chen Z, Zheng ZQ. Study on the controlling strategies of homogeneouscharge compression ignition combustion with fuel of dimethyl ether andmethanol Fuel. 2006;85:204656.

[7] Marriot CD, Reitz RD. Experimental Investigation of Direct Injection-Gasolinefor Premixed Compression Ignited Combustion Phasing Control. SAE Paper,2002-01-0418.

[8] Amano T, Morimoto S, Kawabata Y. Modeling of the Effect of Air/Fuel Ratio andTemperature Distribution on HCCI Engines. SAE Paper, 2001-01-1024.

[9] Richter M, Engstrm J, Franke A, Aldn M, Hultqvist A, Johansson B. TheInfluence of Charge Inhomogeneity on the HCCI Combustion Process. SAE

Paper, 2000-01-2868.[10] Aceves S, Flowers DL, Espinosa-Loza F, Babajimopoulos A, Assanis DN. Analysis

of Premixed Charge Compression Ignition Combustion with a Sequential FluidMechanics-Multizone Chemical Kinetics Model. SAE Paper, 2005-01-0115.

[11] Kumano K, Iida N. Analysis of the effect of charge inhomogeneity on HCCIcombustion by chemiluminescence measurement. SAE Paper, 2004-01-1902.

[12] Naik CV, Pitz WJ, Westbrook CK, Sjberg M, Dec JE, Orme J, Curran HJ, SimmieJM. Detailed Chemical Kinetic Modeling of Surrogate Fuels for Gasoline andApplication to an HCCI Engine. SAE Paper, 2005-01-3741.

[13] Andrae JCG, Bjrnbom P, Cracknell RF, Kalghatgi GT. Autoignition of toluenereference fuels at high pressures modeled with detailed chemical kinetics.Combust Flame 2007;149:224.

[14] Kusaka J, Daisho Y. Multi-dimensional modeling combined with a detailedkinetics (Application for HCCI of Natural Gas). In: Proceedings of the 5thCOMODIA 2001. p. 3306.

[15] FlowersDL, Aceves SM,BabajimopoulosA. Effectof ChargeNon-uniformityon Heat.Release and Emissions in. PCCI Engine. Combustion. SAE Paper, 2006-01-1363.

[16] Peters N, Weber J. The effects of spray formation and evaporation on mixing,auto-ignition and combustion in Diesel engines. In: THIESEL conference onthermo- and fluid dynamic processes in diesel engines; 2006.

[17] Yao MF, Zheng ZL. An Investigation on a New Reduced Chemical Kinetic Modelof n-heptane for HCCI Combustion. Proc Inst Mech Eng D 2006;220(D7):9911002.

[18] .[19] .[20] STAR/KINetics Manual, Reaction Design and CD Adapco Group, 2004.[21] Kong SC, Reitz RD. Numerical study of premixed HCCI engine combustion and

its sensitivity to computational mesh and model uncertainties. CombustionTheory Modeling 2003;7:41733.

[22] Aabo K, Kjiemtrup N. Latest on emission control water emulsion and exhaustgas re-circulation. CIMAC Congress 2004; Paper 126.

[23] Warnatz J, Mass U, Dibble RW. Combustion Physical and ChemicalFundamentals Modeling and Simulation Experiments, Pollutant Formation.3rd ed. Berlin: Springer; 2001.

[24] Aroonsrisopon T, Werner P, Waldman JO, Sohm V, Foster DE, Morikawa T, IidaM. Expanding the HCCI Operation with the Charge Stratification. SAE Paper,2004-01-1756.

[25] Berntsson A, Denbratt I. HCCI Combustion Using Charge Stratification forCombustion Control. SAE Paper, 2007-01-0210.

Z. Zheng, M. Yao/ Fuel 88 (2009) 354365 365
http://www-cms.llnl.gov/combustion/archive.htmlhttp://www.tfd.chalmers.se/~valeri/http://www.tfd.chalmers.se/~valeri/http://www-cms.llnl.gov/combustion/archive.html

Documents

Charge Stratification to Control HCCI