Kinetic Modeling of the Influence of NO on the Combustion Phasing of

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Kinetic modeling of the influence of NO on the combustion phasing of gasoline surrogate fuels in an HCCI engine J.C.G. Andrae Energy Fuels, Just Accepted Manuscript • Publication Date (Web): 15 Oct 2013 Downloaded from http://pubs.acs.org on October 29, 2013

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Kinetic modeling of the influence of NO on the combustion phasing of gasoline surrogate fuels in an HCCI engine J.C.G. Andrae* J A Reaction Engineering, SE-183 32 Täby, Sweden

Abstract In this work a chemical kinetic model describing the nitric oxide (NO) sensitization effect for the oxidation of toluene reference fuels has been developed. The influence of NO on combustion phasing in an HCCI engine has then been studied by kinetic modeling and compared to experiments. An isooctane/n-heptane blend, a toluene/n-heptane mixture and a full boiling range gasoline using a threecomponent surrogate fuel consisting of 55% iso-octane, 23% toluene and 22% n-heptane by liquid volume have been simulated at two different operating conditions with NO concentrations from 4 up to 476 ppm. All three fuels have the same RON of 84. The first operating condition has a high intake pressure (2 bar absolute) and low intake temperature (40°C). The other operating condition has a high intake temperature (100°C) and atmospheric intake pressure. The model predicts in accordance with experimental observations that at high intake pressure in the PRF case the ignition delay is retarded beyond the baseline case (absence of NO in intake) for high concentration of NO while for TRF and the

*

E-mail: [email protected] ACS Paragon Plus Environment

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full boiling range gasoline the combustion phasing is advanced with an increasing NO concentration. The reason for the differences between TRF and PRF fuels is that the promoting effect of toluene is stronger than the one for iso-octane when NO concentration is increased. This is further explained in terms of reaction kinetics. For PRF there is a net production of HONO (nitrous acid) which is chainterminating whereas for TRF there is a net consumption of HONO which is chain-branching. Calculations of fuel sensitivity on the ignition delay time for a gasoline surrogate fuel indicate that it is possible to control the combustion phasing in a gasoline HCCI engine by simultaneously varying the amount of NO (EGR) and the fraction of aromatics and iso-paraffins.

1. Introduction

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Due to concerns regarding the greenhouse effect and to the resulting limitations on carbon dioxide emissions, the possibility of a next-generation combustion mode for internal combustion engines that can simultaneously reduce exhaust emissions and substantially improve thermal efficiency has drawn increasing attention. Homogeneous charge compression ignition (HCCI) is among the most promising solutions in this context. In HCCI engines, the power output is controlled by the fueling rate. Therefore as the load is reduced, in order to achieve high efficiencies, either the mixture becomes leaner or the amount of dilution with exhaust-gas recirculation (EGR) must be increased1. The NO sensitization effect on the auto-ignition can be important in engine operations involving high levels of EGR. This has also been observed for the oxidation of model-fuel components relevant to gasoline, such as n-heptane, iso-octane, and toluene, in stirred reactors as well as in HCCI engines2-6. The effect of NO is a result of the competition between its promoting effect through reaction with HO2 radicals and its retarding effect through reaction with OH radicals, and depends on fuel type and the concentration of the added nitric oxide7. NO tends to increase the extent of oxidation for high-octane fuel components, such as iso-octane and toluene3,6. However, for the low-octane component n-heptane, NO has an inhibiting effect on hydrocarbon oxidation, particularly at low temperatures corresponding to the negative temperature coefficient (NTC) region3. In HCCI engine studies, Dubreuil et al4 showed that the promoting effect on the ignition delay of nheptane is maximal at 100 ppmv NO and then less effective for higher concentrations. The increase in ignition delay and the behavior with NO addition was the same for the EGR rates tested. Risberg et al2 found that at boosted intake pressure (2 bar absolute) and low intake temperature, a PRF 84 fuel exhibited a longer ignition delay time for high concentrations of NO in the intake than the baseline case (0 ppm NO) but the effect was much less pronounced at atmospheric intake pressure and high intake temperature. On the other hand, for the studied TRF 84 the ignition delay decreased with increased NO in the intake for both operating conditions. Moreover, the results for the TRF 84 showed better similarities to the full-boiling range gasoline with the same RON. Contino et al6 showed that at atmospheric intake pressure and high intake temperature, when using neat iso-octane as fuel the ignition ACS Paragon Plus Environment

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delay monotonically decreased when NO increased up to 500 ppmv. They assumed that the different behaviors seen for iso-octane and n-heptane could be explained by the dominant inhibiting impact of NO on n-heptane at low temperature8 which is less significant for iso-octane. For improving the understanding of the NO sensitization effect on the auto-ignition of fuel mixtures, it is important for a chemical kinetic model to be able to predict the influence of NO on the combustion phasing of gasoline surrogate fuels for different operating conditions and fuel compositions. Previous modeling studies using PRF and TRF as surrogate fuels have mostly been limited to cases with atmospheric intake pressure and to single- and two-component fuel blends4-6,8,9. However, simulations of the NO sensitization effect for TRF at boosted intake pressure have not been presented. Auto-ignition chemistry is influenced by both pressure and temperature. At boosted intake pressures the first stage or cool flame heat release is much more prominent compared to atmospheric intake pressure. Cool flameand high temperature ignition delays are influenced by the concentration of the added nitric oxide as well as by the fuel nature. The main topic of this paper is a better understanding for these conditions, particularly for TRF and for the interaction of NO and aromatic fuels. A semi-detailed TRF mechanism developed in10 has been extended to incorporate the sensitization effect of NO on the oxidation. The mechanism has been validated against available jet stirred reactor measurements for n-heptane, iso-octane and toluene with and without added NO, and also against well defined CAI experiments for a three-component toluene surrogate fuel. The validated mechanism is then used in kinetic modeling of HCCI experiments using a single-zone model. The NO sensitization effect on the auto-ignition for two operating conditions (high- and low intake pressure) and two gasoline surrogate fuels is examined2. Also a full boiling range gasoline is modeled using a three-component surrogate fuel. The results are further examined with brute forced sensitivity and species rate-of-production analyses and explained in terms of reaction kinetics. 2. Chemical kinetic model


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The starting mechanism for model development was a semi-detailed chemical reaction mechanism for toluene reference fuels10 involving 137 species and 635 reactions (134 species and 613 reactions after removal of a small set of NO reactions). It consists of a detailed reaction mechanism for toluene, and skeleton mechanisms for the combustion of the primary reference fuels (iso-octane and n-heptane). The semi-detailed TRF mechanism has been extended in this work to incorporate the sensitization effect of NO. First the NO sub-mechanism with thermodynamic data of GRI-MECH 3.0 was added11 which consists of 17 species and 106 reactions. It is known that the fate of nitrous acid (HONO) plays an important part in the sensitization effect of NO on hydrocarbon oxidation7. To include updated reactions involving HONO a sub-set by Naik et al12 was also incorporated in the mechanism. Table 1 shows the reactions with associated rate constants for the HONO sub-set. The last step in the construction of the mechanism was to add reactions that describe interactions of TRF and NO. These were based on the work by Anderlohr et al8 and are shown in Table 2. As the model in8 is very detailed, some of the reactions and rate constants had to be modified to fit with the semi-detailed TRF mechanism used in this work. After the addition of the three sub-sets, the final TRF-NO mechanism consists of 155 species and 777 reactions. The full mechanism is given in electronic format as Supporting Information to the article. The results from the validation of the mechanism are described in Section 4.

3. Numerical solution method

All simulations were performed with the CANTERA software package13. The CSTR model in CANTERA was used to model jet-stirred reactor experiments. For simulations of ignition delay at adiabatic conditions, a homogeneous reactor was assumed with a constant internal energy, constant volume constraint. Ignition was assumed to be accomplished at the moment of maximum pressure rise rate. Similar ignition delay time (for high-temperature ignition) is achieved when having an ignition criterion as the time when the temperature has increased 400 K from the initial temperature.


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Based on the geometrical engine data, compression ratio and engine speed, the HCCI combustion experiments were modeled using a single-zone model. The variation of specific volume as a function of time could be with negligible error ( OH+2CH2O+

300 ppm NO

755. O2C7H14O2H+NO -> OH +2CH2O+CH2CHCH3+C2H4+NO2

IC4H8+C2H4+NO2

-4

-2

0

2

4

6

S (τ )/ %

Figure 16. Brute force sensitivity analysis for Fuel A (PRF 84). Adiabatic constant volume reactor. To=740 K, po=28 bar, λ=4.

Fig. 17 shows the results for the TRF at the same initial conditions. There are significant differences compared to the PRF with the most important one being for reaction 729 which shows a negative sensitivity for both low and high NO concentration. Also reaction 766 shows a large negative sensitivity while the PRF does not show sensitivity toward the corresponding reactions 747-749. In addition to having a large negative sensitivity in reaction 729, a rate of production analysis at the conditions of Fig. 17 indicates that there is a net consumption of HONO for the TRF at both NO concentrations. Reaction 766, which proceeds in the reverse direction, is the largest producer of HONO which is consumed to a large extent in reaction 729. This is chain-branching and explains the decreased high temperature ignition delay for the TRF for increasing NO concentrations. This would also explain why the full boiling range gasoline and the gasoline surrogate fuel show a similar behavior to the TRF as these fuels contain significant amounts of toluene.


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15 ppm NO

622. HO2+NO NO2+OH 624. NO2+O NO+O2 625. NO2+H NO+OH 724. NO2+HO2 HNO2+O2 729. NO+OH(+M) HONO(+M) 730. NO2+HO2 HONO+O2 732. HONO+OH NO2+H2O

300 ppm NO

755. O2C7H14O2H+NO ->OH+ 2CH2O+CH2CHCH3+C2H4+NO2

766. C6H5CH2+HONO C6H5CH3+NO2 769. C6H5CH2+NO2 C6H5CH2O+NO 770. C6H5CH2OO+NO NO2+C6H5CH2O

-6

-4

-2

0

2

4

S (τ ) / %

Figure 17. Brute force sensitivity analysis for Fuel B (Toluene 65%/n-heptane 35%). Adiabatic constant volume reactor. To=740 K, po=28 bar, λ=4.

In a further analysis, the effect of compositional changes in the gasoline surrogate fuel in section 4.2 has been considered for the same initial conditions. The base composition has been varied by doping it with an additional 10% (absolute) of each of the three fuel components19. LTHR and HTHR delay times have been extracted from the pressure histories as the moment of maximum pressure rise rate and the sensitivity to the doped component (X) is calculated as

S ( LTHR, HTHR ) =

τ ( Base + 10% X ) − τ ( Base ) × 100 τ ( Base )


(13)

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Fig’s 18 and 19 show the obtained sensitivity coefficients for LHTR and HTHR and different NO concentrations. As expected increased n-heptane content would give shorter LTHR and HTHR ignition delays (large negative sensitivity), however those are not affected much by the addition of NO at the studied conditions.

n-Heptane 300 ppm n-Heptane 15 ppm n-Heptane 0 ppm iso-Octane 300 ppm iso-Octane 15 ppm iso-Octane 0 ppm Toluene 300 ppm Toluene 15 ppm Toluene 0 ppm

-10

0

10

20

30

S (LTHR) / %

Figure 18. Fuel sensitivity in a gasoline surrogate fuel (55% iso-octane, 23% toluene, 22% n-heptane) to cool flame ignition delay for different initial NO concentrations. To=740 K, po=28 bar, λ=4.


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n-Heptane 300 ppm n-Heptane 15 ppm n-Heptane 0 ppm iso-Octane 300 ppm iso-Octane 15 ppm iso-Octane 0 ppm Toluene 300 ppm Toluene 15 ppm Toluene 0 ppm

-20

0

20

40

S (HTHR) / %

Figure 19. Fuel sensitivity in a gasoline surrogate fuel (55% iso-octane, 23% toluene, 22% n-heptane) to high temperature ignition delay for different initial NO concentrations. To=740 K, po=28 bar, λ=4.

Doping the fuel with iso-octane would decrease the ignition delay, but the sensitivity is low compared to n-heptane as expected. An increased concentration of NO has a retarding effect on both LTHR and HTHR. This is partly explained by the presence of toluene which has a stronger sensitivity towards NO addition than iso-octane although the fraction of iso-octane in the fuel mixture is higher. Toluene shows the largest sensitivity of the three fuel components, where doping would increase both the LTHR and HTHR delay as opposed to n-heptane and iso-octane which show an accelerating effect on both ignition delays. However, an increased NO concentration has an accelerating effect on the LTHR and the HTHR for higher NO concentrations. The ratio between S(HTHR) and S(LTHR) is not changed much for n-heptane for different amounts of NO while it is changed more significantly for iso-octane and toluene. This indicates that it is possible to control the combustion phasing in a gasoline HCCI engine by simultaneously varying the amount of NO (EGR) and the fraction of aromatics and iso-paraffins.


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6. Conclusions

A chemical kinetic model describing the nitric oxide (NO) sensitization effect for the oxidation of toluene reference fuels has been developed.

The influence of NO on combustion phasing in an HCCI engine has then been studied by kinetic modeling and compared to experiments. The model predicts in accordance with experimental observations that at high intake pressure in the PRF case the ignition delay is retarded beyond the baseline case (absence of NO in intake) for high concentration of NO while for TRF and the full boiling range gasoline the combustion phasing advanced with an increasing NO concentration.

The reason for the differences between TRF and PRF fuels is that the promoting effect of toluene is stronger than the one for iso-octane when NO concentration is increased. This is further explained in terms of reaction kinetics. For PRF there is a net production of HONO (nitrous acid) which is chainterminating whereas for the TRF there is net consumption of HONO which is chain-branching.

Calculations of fuel sensitivity on the ignition delay time for a gasoline surrogate fuel indicate that it is possible to control the combustion phasing in a gasoline HCCI engine by simultaneously varying the amount of NO (EGR) and the fraction of aromatics and iso-paraffins.

Nomenclature

A(t)

Instantaneous Area for heat transfer

m2

B

Bore

m

C

Compression Ratio

-


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CA ATDC

Crank Angle Degrees After Top Dead Center

-

CAI

Controlled Auto Ignition

h(t)

Instantaneous Heat transfer coefficient

HCCI

Homogeneous Charge Compression Ignition

HTHR

High Temperature Heat Release

J/CAD

IMEP

Indicated Mean Effective Pressure

bar

JSR

Jet Stirred Reactor

Lc

Connecting Rod Length

m

LTHR

Low Temperature Heat Release

J/CAD

N

Engine Speed

rev min-1

p(t)

Instantaneous Cylinder Pressure

bar

PRF

Primary Reference Fuels

q

Heat Release Rate

J/CAD

qwall

Heat flux to wall

W

RON

Research Octane Number

S

Stroke

m

Sp

Mean Piston Speed

ms-1

T(t)

Instantaneous cylinder temperature

K

Tw

Wall Temperature

K

TRF

Toluene Reference Fuels

V(t)

Instantaneous Cylinder Volume

m3

Vc

Cylinder Clearance Volume

m3

Wm-2K-1

Greek letters


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γ

Specific heat ratio

-

θ

Crank Angle Degree

-

τ

Ignition Delay

CAD, s

λ

Excess air-ratio

-

φ

Fuel-air equivalence ratio

-

References (1) Dec, J.E. Proc. Combust. Inst. 2009, 32, 2727-2742. (2) Risberg, P.; Johansson, D.; Andrae, J.; Kalghatgi, G.; Björnbom, P.; Ångström, H.E. SAE Technical Paper 2006, doi:10.4271/2006-01-0416. (3) Moréac, G.; Dagaut, P.; Roesler, J.F.; Cathonnet, M. Combust. Flame 2006, 145, 512-20. (4) Dubreuil, A.; Foucher, F.; Mounaim-Rousselle, C.; Dayma, G.; Dagaut, P. Proc. Combust. Inst. 2007. 31. 2879–86. (5) Machrafi, H; Guibert, P.; Cavadias S. Combust. Sci. Tech. 2008, 180, 1245-1262. (6) Contino, F.; Foucher, F.; Dagaut, P.; Lucchini, T.; D’Errico, G.; Mounaïm-Rousselle, C. Combust. Flame 2013, 160, 1476-1483. (7) Prabhu, S.K.; Bhat, R.K.; Miller, D.L.; Cernansky, N.P. Combust. Flame 1996, 104, 377-390. (8) Anderlohr, J.M.; Bounaceur, R.; Pires Da Cruz, A.; Battin-Leclerc, F. Combust. Flame 2009, 156, 505-521. (9) Naik, C.V.; Puduppakkam, K.; Meeks, E. SAE International Journal of Fuels and Lubricants 2010, 3, 556-566. (10) Andrae, J.C.G. Fuel 2013, 107, 740-48. ACS Paragon Plus Environment

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(11) GRI-MECH 3.0, http://www.me.berkeley.edu/gri_mech/ (12) Naik, C.V.; Puduppakkam, K.V.; Modak, A.; Meeks, E.; Wang, Y.L.; Feng, Q.; Tsotsis, T.T. Combust. Flame 2011, 158, 434-445. (13) CANTERA 1.7, http://sourceforge.net/projects/cantera, Accessed: June 1, 2010. (14) Hohenberg, G.F. SAE Technical Paper 1979, doi:10.4271/790825. (15) Heywood, J.B. Internal combustion engines fundamentals. McGraw-Hill, Inc., 1988. (16) Knop, V.; Pera, C.; Duffour, F. Combust. Flame 2013, 160, 2067-2082. (17) Pera, C.; Knop, V. Fuel 2012, 96, 59-69. (18) Knop, V.; Loos, M.; Pera, C.; Jeuland, N. Fuel 2014, 115, 666-673. (19) Kukkadapu, G.; Kumar, K.; Sung, C.J.;, Mehl, M.; Pitz, W.J. Combust Flame 2012, 159, 3066-78.


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Table 1. Reactions and rate constants for the nitrous acid (HONO) sub-set based on Naik et al.12.

Number 720

Reaction HNO2(+M)=HONO(+M) Low pressure limit: TROE centering:

3.100E+18 1.149E+00

0.00E+00 1.000E-30

A

N

2.500E+14 3.150E+04 3.125E+03

0.00 1.0E+30

Ea 32300

721

HNO2+O=NO2+OH

1.700E+08

1.50

722

HNO2+OH=NO2+H2O

4.000E+13

0.00

723

NO2+H2=HNO2+H

2.430E+00

3.73

32400

724

NO2+HO2=HNO2+O2

1.850E+01

3.26

4983

725

NO2+CH4=HNO2+CH3

6.000E+14

0.00

37600

726

NO2+CH2O=HNO2+HCO

1.070E-01

4.22

19850

727

NO2+CH3OH=HNO2+CH2OH

2.410E+03

2.90

27470

728 729

NO2+C2H6=HNO2+C2H5 NO+OH(+M)=HONO(+M) Low pressure limit: TROE centering:

6.000E+14 1.100E+14 0.000E+00 1.000E+30

0.00 -0.30

33200 0

1.0E+30

2.35E+23 0.81

-2.40E+00 1.00E-30

2000

730

NO2+HO2=HONO+O2

1.910E+00

3.32

3044

731

HONO+O=NO2+OH

1.200E+13

0.00

5960

732

HONO+OH=NO2+H2O

1.700E+12

0.00

-520

733

2HONO=NO+NO2+H2O

3.493E-01

3.64

12140

734

NO2+H2=HONO+H

1.300E+04

2.76

29770

735

NO2+CH4=HONO+CH3

6.500E+14

0.00

45800

736

NO2+HCO=HONO+CO

4.950E+12

0.00

0

737

NO2+CH2OH=HONO+CH2O

5.000E+12

0.00

0

738

NO2+CH3O=HONO+CH2O

6.000E+12

0.00

2285

739

NO2+CH3OH=HONO+CH2OH

1.450E+02

3.32

20035

740

NO2+C2H6=HONO+C2H5

6.500E+14

0.00

41400

741

CN+HONO=HCN+NO2

1.200E+13

0.00

0

742

NCO+HONO=HNCO+NO2

3.600E+12

0.00

0

743

NH2+HONO=NH3+NO2

7.100E+01

3.02

-4940

744

NH+HONO=NH2+NO2

1.000E+13

0.00

0

745

HONO+H=HNO+OH

5.600E+10

0.86

5000

746

HONO+H=NO+H2O

8.100E+06

1.89

3850

The number in the first column corresponds to the reaction number in the mechanism. A units = cm3 mol, s. Ea unit = cal/mol. kforward = A T n e (- Ea/RT).


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Energy & Fuels

Table 2. Reactions and rate constants for the TRF/NO sub-set based on the work of Anderlohr et al8. Number

Reaction

A

n

Ea

RH + NO2 = R + HONO 747

C7H16+NO2=C7H15-1+HONO

1.32E+14

0.00

31100

748

C7H16+NO2=C7H15-2+HONO

5.80E+13

0.00

28100

749

C8H18+NO2=AC8H17+HONO

3.30E+14

0.00

31100

R + NO2 => R' + aldehyde + NO a

C7H15-1+NO2=>C5H11+CH3HCO+NO

2.00E+13

0.00

15000

a

C7H15-2+NO2=>C5H11+CH3HCO+NO

2.00E+13

0.00

15000

a

AC8H17+NO2=>C5H11+C2H5CHO+NO

2.00E+13

0.00

15000

750 751 752

ROO + NO => R' + aldehyde + NO2 753

C7H15O2+NO=>C5H11+CH3HCO+NO2

2.00E+13

0.00

15000

754

AC8H17OO+NO=>C5H11+C2H5CHO+NO2

2.00E+13

0.00

15000

OOQOOH + NO => OH + 2 CH2O + olefin + NO2 755

O2C7H14O2H+NO=>OH+2CH2O+CH2CHCH3+C2H4+NO2

4.70E+12

0.00

-358

756

AC8H16OOH-BO2+NO=>OH+2CH2O+IC4H8+C2H4+NO2

4.70E+12

0.00

-358

aldehyde + NO2 => R + CO + HONO 757

CH3HCO+NO2=>CH3+CO+HONO

8.35E-11

6.68

8300

758

C2H5CHO+NO2=>C2H5+CO+HONO

8.35E-11

6.68

8300

Reactions of benzene and phenyl radicals 759

C6H6+NO2=C6H5+HONO

7.41E+13

0.00

38200

760

C6H6+NO2=C6H5+HNO2

2.50E+14

0.00

42200

761

C6H5+HNO=C6H6+NO

3.78E+05

2.28

456

762

C6H5+HNO=C6H5NO+H

3.79E+09

1.19

95400

763

C6H5NO2=C6H5+NO2

1.52E+17

0.00

73717

764

C6H5NO=C6H5+NO

1.52E+17

0.00

55200

Reactions of toluene and benzyl radical a

C6H5CH2+HNO2=C6H5CH3+NO2

2.50E+04

3.00

12700

a

C6H5CH2+HONO=C6H5CH3+NO2

2.50E+04

3.00

13000

767

C6H5CH2+HNO=C6H5CH3+NO

1.47E+10

0.76

349

768

C6H4CH3+HNO=C6H5CH3+NO

3.78E+05

2.28

456

769

C6H5CH2+NO2=C6H5CH2O+NO

1.36E+12

0.00

0

2.35E+12

0.00

-358

765 766

Reaction of benzylperoxy radicals a

770

C6H5CH2OO+NO=NO2+C6H5CH2O Reactions of benzylalkoxy radicals

771

C6H5CH2O+NO=C6H5CHO+HNO

7.60E+13

-0.76

0

772

C6H5CH2O+NO2=C6H5CHO+HONO

4.00E+12

0.00

2285

773

C6H5CH2O+HNO=C6H5CH2OH+NO

3.16E+13

0.00

0

Reactions of benzaldehyde 774

C6H5CHO+NO2=C6H5CO+HONO

8.35E-10

6.68

8300

775

C6H5CHO+NO=C6H5CO+HNO

1.02E+13

0.00

40670

Reactions of nitrobenzene


33

Energy & Fuels

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 34 of 34

776

C6H5NO2=C6H5O+NO

7.12E+13

0.00

62590

777

C6H5NO+NO2=C6H5NO2+NO

9.62E+13

0.00

12928

The number in the first column corresponds to the reaction number in the mechanism. A units = cm3 mol, s. Ea unit = cal/mol. kforward = A T n e (- Ea/RT). a

Rate constant revised in this work.


34

Kinetic Modeling of the Influence of NO on the Combustion Phasing of

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