2004 fuel zervas e. influence of fuel and air fuel equivalence ratio on the emission of hydrocarbons...

7/30/2019 2004 Fuel Zervas E. Influence of Fuel and Air Fuel Equivalence Ratio on the Emission of Hydrocarbons From a SI E

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Influence of fuel and air/fuel equivalence ratio on the emissionof hydrocarbons from a SI engine.

2. Formation pathways and modelling of combustion processes

E. Zervasa,*, X. Montagnea, J. Lahayeb

aInstitut Francais du Petrole, 1 et 4 avenue du Bois Preau, Rueil-Malmaison cedex F-92500, FrancebInstitut de Chimie d es Surfaces et Interfaces, 15 rue Jean Starcky, Mulhouse cedex F-68057, France

Received 19 January 2004; revised 22 June 2004; accepted 24 June 2004

Available online 7 August 2004

Abstract

A spark ignition engine was used to study the impact of fuel composition and of the air/fuel equivalence ratio on exhaust emissions of

specific hydrocarbons. The fuel blends used contained eight main hydrocarbons and four oxygenated compounds. The identification of each

exhaust pollutant fuel precursor is already done. After this identification, several models correlating the exhaust concentration of these

pollutants with the fuel composition are presented on each air/fuel equivalence ratio. Based on the above findings, the main formation paths

for the formation of each exhaust pollutant are proposed.

q 2004 Elsevier Ltd. All rights reserved.

Keywords: Exhaust Emissions; Modelling; Specific Hydrocarbons

1. Introduction

Correlations between fuel composition and exhaust

emissions from spark ignition (SI) engines have been

extensively researched for regulated pollutants ([14] and

many others). However, exhaust hydrocarbons are a sum of

several species, and the emissions of individual hydro-

carbons have not been thoroughly investigated yet.

In three previous works, we presented the influence of

fuel and ofl on the emission of organic acids [5], alcohols

and carbonyl compounds [6], and regulated pollutants (CO,

total HC and NOx

[7]) of a SI engine. Continuing this work,

the influence of the above parameters on the emission ofspecific hydrocarbons is now presented in two papers. The

first one [8] presents the experimental findings of this

study: the main precursors of each pollutant, the influence of

air/fuel equivalence ratio, the percentage of each pollutant

over total emitted HC, the relations with the fuel physicalproperties, the correlations of the emitted HC with other

exhaust components, etc. Based on the results presented on

the first article, the second one is firstly focusing on the

modelling between the emitted pollutants and fuel compo-

sition and secondly on the formation pathways of these

pollutants.

2. Experimental section

The experimental procedure is presented in the first

article of this study [8] and elsewhere [9].

3. Results and discussion

3.1. Models

A quantitative model relating the exhaust concentration

(in ppmv) with the contents of the fuel components (in % of

volume) is searched for each pollutant: Exhaust pollutantZ

a!fuel component1Cb!fuel component2C/ (Table 1).

All models presented here take also into account

0016-2361/$ - see front matter q 2004 Elsevier Ltd. All rights reserved.

doi:10.1016/j.fuel.2004.06.028

Fuel 83 (2004) 23132321

www.fuelfirst.com

* Corresponding author. Address: Renault-CTLL26060, 1, Allee Cor-

nuel, F-91510 Lardy, France. Tel.: C331-6927-8477; fax: C331-6927-

8292.

E-mail address: [email protected] (E. Zervas).
http://www.fuelfirst.com/http://www.fuelfirst.com/


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the composition of alkylate. Using all experimental points,

the lines Predicted valuesZa1!experimental valuesCb1are estimated. The r2 of the presented models are always

higher than 0.91, with a1 very close to 1.0 and b1 very close

to 0.0, indicating a very good accordance between predicted

and experimental values.

As methane and ethane are the final products of a seriesof reactions, their exhaust concentration is not directly

linked with the fuel composition and no such model is

found.

According to the ethylene model at stoichiometry, the

majority of this pollutant comes from fuel octane (25.3%),

hexane (19.6%), 1-hexene (18.2%) and cyclohexane

(15.3%), followed by isooctane, ETB, 2-propanol and

o-xylene. In accordance with the first article [8], the main

sources of ethylene are the straight chain hydrocarbons. The

percentage of hexane and octane decreases with l, this of

isooctane, o-xylene and ETB increases, while this of

1-hexene, cyclohexane and 2-propanol presents a maximum

at lZ1.0. The increasing percentage at rich conditions

indicates that the corresponding fuel components are

cracking easier to give ethylene than the others under

these conditions.

At stoichiometry, the majority (80%) of acetylene comes

from fuel benzene. As benzene is found in almost equal

content in all fuels, exhaust concentration of acetylene is

almost equal in all fuels [8]. All other sources are less

important, their participation is 0.83.4% each. The

percentage of benzene decreases with lambda (from

88.5% at lean conditions to 62.6% at rich ones), while this

of all other compounds increases.

The most important source of propylene at stoichiometryis fuel isooctane (24.1%), followed by 2-propanol (23.3%),

1-hexene (20.0%), octane (15.6%), hexane (13.3%) and

cyclohexane (3.7%). The percentage of 2-propanol and

isooctane presents a maximum at stoichiometry, this of

hexane, cyclohexane and octane a minimum at the same

point, while this of 1-hexene increases.

A model linking the exhaust concentration of isobutane

with fuel isooctane is constructed, but it is not very good,

the r2 of the line Predicted valuesZa1!experimental

valuesCb1, is only 0.743 (a1Z0.78 and b1Z0.44). The

addition of MTBE or of a constant, or even of other fuel

components, does not improve it, indicating that this

pollutant must also have other sources not directly linked

with the initial fuel composition.

At stoichiometry, exhaust 1-butene is formed from fuel

octane (36.3%), hexane (32.8%), hexene (21.2%) and

isooctane (9.6%). The percentage of hexane decreases at

rich conditions, this of isooctane and hexene presents a

maximum at stoichiometry, while this of octane presents a

minimum at the same point.

Most of the half (55.6%) of exhaust isobutene comes

from fuel MTBE and the rest from isooctane at lZ1.0,

while the addition of other fuel components does not

improve this model. The percentage of isooctane increases

at rich conditions, while this of MTBE decreases. Contrary

to isobutane, MTBE clearly participates to the formation of

isobutene.

As the exhaust concentrations of cis and trans-2-butene

are very low, no model linking them with fuel components

is found.

At stoichiometry, the quasi totality of exhaust isopentanecomes from fuel isopentane, but isooctane also contributes

by 1.01.7%. Lambda does not seem to influence these

contributions. The addition of other fuel components does

not improve this model, indicating that fuels isopentane

and isooctane are the only sources of this pollutant.

At stoichiometry, the majority (82%) of exhaust benzene

comes from fuel benzene; the other sources are ETB,

toluene, o-xylene and cyclohexane. These percentages

change with l: the specific weight of benzene decreases at

rich conditions, this of the other three aromatics increases

because of the dealkylation increase, while this of

cyclohexane decreases. This model shows that the specific

weight of fuel benzene is the most important, but the

quantity of exhaust benzene of a commercial fuel

comes principally from the other aromatics due to their

high content in the fuel. Kameoka [10] presents the model

BZ0.56BC0.05TC0.04XC0.08ETB (in C basis).

In the case of exhaust toluene, the two-thirds of its

concentration at stoichiometry come from fuel toluene; fuel

o-xylene contributes with 26% and the rest comes from fuel

ETB. Benzene is not statistically significant for this model.

These percentages change with l; the specific weight of

toluene decreases at rich conditions, this of ETB globally

increases, while this of o-xylene presents a maximum at

stoichiometry. Kameoka [10] presents that exhaust tolueneis linear with fuel xylenes and ETB.

At lZ1.0, the 85% of exhaust ETB comes from fuel

ETB; the rest comes from toluene and o-xylene. These

percentages change with l; the specific weight of toluene

and o-xylene presents a maximum at stoichiometry, while

this of ETB presents a minimum value at the same point.

The benzene is not statistically significant to this

model. Kameoka [10] presents that exhaust ETB is linear

with fuel ETB.

In a previous article [9], we proposed four models linking

the exhaust concentration of 1,3-butadiene with fuel

1-hexene and cyclohexane, the concentration of exhaust

1-hexene with fuel 1-hexene and cyclohexane, the concen-

tration of exhaust cyclohexane with fuel cyclohexane and

the concentration of isopropylbenzene with fuel ETB. A

closer analysis shows that four improved models are valid.

At stoichiometry, more than the half of exhaust

1,3-butadiene comes from fuel cyclohexane (50.5%); the

other sources are 1-hexene (19.7%), o-xylene (11.3%),

octane (10.7%), isooctane (4.0%) and hexane (3.8%).

The percentage of cyclohexane increases at rich conditions

(from 31 to 61%), this of 1-hexene and octane decreases,

while this of o-xylene, hexane and isooctane presents a

maximum at stoichiometry. In the second case, at

E. Zervas et al. / Fuel 83 (2004) 231323212314


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Table 1

Model coefficients for the five air/fuel equivalence ratios used. Percentages of participation of each fuel component and r2, a1 and b1 of the lines Predicted

valuesZa1!experimental valuesCb1

Pollutant, l Fuel component

r2 a1 B1 C6 C6Z CC6 C8 iC8 B T o-X ETB P MTBE

Ethylene 0.917 0.979 4.338

1.25 23.3 18.7 14.4 30.4 5.1 2.9 4.4 0.81.11 21.1 18.2 14.9 28.6 5.7 3.2 5.6 2.7

1 19.6 18.2 15.3 25.3 6.9 3.6 5.6 5.5

0.91 17.3 18.5 15.3 27.2 8.3 4.3 6.5 2.7

0.83 16.5 17.4 13.6 28.0 8.3 7.3 7.1 1.8

Acetylene 0.967 0.967 1.576

1.25 0.9 1.4 1.8 1.3 1.6 88.5 0.1 1.8 2.6

1.11 1.3 1.6 2.1 2.4 2.2 84.6 0.4 2.3 3.1

1 2.0 2.4 3.2 3.4 3.1 79.7 0.8 2.3 3.1

0.91 2.5 3.0 4.0 3.8 3.8 75.6 1.6 2.7 3.1

0.83 3.9 4.6 5.6 5.6 4.7 62.6 2.4 4.8 5.8

Propylene 0.940 0.950 0.948

1.25 22.7 17.3 4.7 23.7 19.2 12.4

1.11 19.4 18.2 4.5 21.4 20.7 15.8

1 13.3 20.0 3.7 15.6 24.1 23.3

0.91 13.9 25.9 5.5 22.8 27.2 4.60.83 16.4 25.9 6.4 23.3 24.0 4.1

1-Butene 0.943 0.970 0.021

1.25 51.9 1.9 0 45.2 1.0

1.11 46.1 7.3 0.8 44.0 2.6

1 32.8 21.2 2.2 36.3 9.6

0.91 31.8 20.5 4.3 38.9 8.8

0.83 31.9 19.3 5.9 42.1 6.8

Isobutene 0.953 1.04 1.07

1.25 42.2 57.8

1.11 42.8 57.2

1 44.4 55.6

0.91 47.2 52.8

0.83 48.6 51.4

1,3-Butadiene 0.940 0.958 0.1101.25 0.0 36.7 31.5 22.1 3.7 6.0

1.11 0.9 32.6 34.2 21.6 3.9 6.8

1 3.8 19.7 50.5 10.7 4.0 11.3

0.91 3.3 17.2 57.9 9.4 3.7 8.5

0.83 1.9 17.9 61.2 9.1 3.0 6.9

1-Hexene 0.953 0.909 0.273

1.25 64.83 27.14 8.03

1.11 68.01 24.00 7.99

1 71.07 27.20 1.72

0.91 71.8 27.81 0.39

0.83 71.56 28.44 0

Cyclohexane 0.965 0.982 0.259 75.7 4.0 5.1 7.5 7.8

1.25 78.4 3.9 4.8 6.4 6.6

1.11 80.4 3.3 4.5 5.5 6.2

1 85.6 2.7 3.1 4.2 4.50.91 88.6 2.2 2.5 3.4 3.2

0.83

Benzene 0.95 0.96 0.23 4.7 88.1 3.9 1.5 1.7

1.25 4.8 84.3 3.9 3.6 3.4

1.11 2.2 81.7 4.9 4.8 6.5

1 1.4 77.4 7.1 5.6 8.7

0.91 1.3 71.1 8.3 7.4 12.0

0.83

Toluene 0.979 0.99 K0.09 89.9 7.9 2.2

1.25 83.7 12.3 4.0

1.11 66.8 25.9 7.3

1 61.1 20.5 18.4

(continued on next page)

E. Zervas et al. / Fuel 83 (2004) 23132321 2315


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stoichiometry, the majority (71%) of exhaust 1-hexene

comes from fuel 1-hexene, 27% from fuel cyclohexane,

but a small quantity (2%) from fuel octane. The addition

of hexane does not change this model. The percentage

of 1-hexene increases at rich conditions, the one of

cyclohexane remains almost constant, while the percentage

of octane decreases from 8 to 0%. In the case of

cyclohexane, a closer analysis shows that the addition of

fuel aromatics improves this model for every l used. At

stoichiometry, the majority (80%) of cyclohexane comes

from fuel cyclohexane, but aromatics contribute with

36% each. The specific weight of cyclohexane increases

at rich conditions, while this of aromatics at lean ones.

According to the improved model, at stoichiometry, the

81% of exhaust iPB comes from fuel ETB; fuel o-xylene

contributes by 15% and the rest 4% comes from fuel

toluene. These percentages change with l; the specific

weight of toluene is more important at lean conditions, the

percentage of ETB presents a minimum value at lZ1.0,

while o-xylene participates only at rich conditions.Benzene does not participate to this model.

Pentane, hexane, octane, isooctane and o-xylene come

only from the respective fuel components. The addition of

other fuel components does not improve these correlations.

The r2 of the lines Predicted valuesZa1!experimental

valuesCb1 are 0.934, 0.979, 0.971, 0.978 and 0.980,

respectively, for the above models, indicating a very

good accordance between predicted and experimental

values (the a1 are 0.923, 0.985, 0.951, 0.978 and 0.960

and the b1 are 0.038, 0.001, 0.202, 0.190 and K0.84,

respectively).

3.2. Formation paths

Methane is the lighter hydrocarbon and its formation is at

the end of the cracking path of all the other HC. It is clearly

enhanced by some hydrocarbons that obviously can produce

more CH3 radicals than the others (for example o-xylene or

isooctane), but normally it can be formed from all fuel

components. The same conclusions are valuable in the case

of ethane; even if it is clearly enhanced by some fuel

components, it can be produced from many hydrocarbons by

b-scissions.

Ethylene. This pollutant is a product of b-scission of

higher HC. Octane is the main source of ethylene, because it

can easily proceed to its formation by three b-scissions

(Fig. 6); hexane (Fig. 1) and 1-hexene (Fig. 2) can proceed

by two. The contribution of cyclohexane is smaller because

it must firstly proceed by a ring opening (Fig. 3). Isooctane

can produce C2 products at the end of its combustion, but

as this path is more difficult than the b-scissions of the

above fuel components, its contribution is lower. ETBcan give ethylene after a dealkylation of the ethyl radical

[6], but the quantity of ethylene produced is smaller than

this of the other sources. Other aromatics can produce this

compound after a ring opening, formation of unsaturated C2radicals and combination with H, but this path seems quite

difficult.

Acetylene. Zhang [11] studies the combustion of benzene

and proposes that the formation of acetylene is found on the

main path of its oxidation: C6H60C6H50C4H30C2H2.

The other aromatics can also follow the same mechanism

after a dealkylation. Acetylene can also be produced from

Table 1 (continued)

Pollutant, l Fuel component

r2 a1 B1 C6 C6Z CC6 C8 iC8 B T o-X ETB P MTBE

0.91 57.5 20.1 22.4

0.83

ETB 0.980 0.98 0.09 0.4 2.3 97.3

1.25 2.7 2.8 94.51.11 6.9 8.0 85.1

1 3.9 3.6 92.5

0.91 1.8 2.3 95.9

0.83

o-Xylene 0.980 0.96 K0.84 100

1.25 100

1.11 100

1 100

0.91 100

0.83

iPB 0.985 0.96 0.11 4.5 0 95.5

1.25 4.4 0 95.6

1.11 4.1 15 80.7

1 2.6 13 84.5

0.91 1.3 12 86.9

0.83



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all the other hydrocarbons by b-scissions followed by H

extraction, but its formation after this mechanism, is more

difficult than that of ethylene; this is the reason of its lower

concentration comparing to ethylene.

Propylene: from 2-propanol (Fig. 4). This compound

loses its OH and the corresponding radical is stabilized

losing an H. From isooctane (Fig. 5). Isooctane can initially

break in two points and form either two C4 radicals or a C3and a C5 [12]. The C3 radical loses an H to form propylene.

Another path is the formation of an tertiary isooctyl radical

[13], which is decomposed to a methyl radical and

propylene [14]. From 1-hexene (Fig. 2). 1-Hexene can

initially give a C2 and a C4 radical but also two C3 ones. The

primary C3 radical can give a secondary one by internal

arrangements and propylene after an H extraction. The

unsaturated C3 radical can combine with an H to form

Fig. 1. Proposed path for the hexane combustion and formation of ethylene,

propylene and 1-butene.

Fig. 2. Proposed path for the 1-hexene combustion and formation of

ethylene, propylene and 1-butene.

Fig. 3. Proposed path for the cyclohexane combustion and formation of

ethylene, propylene, 1,3-butadiene and 1-hexene.

Fig. 4. Proposed path for the 2-propanol combustion and formation of

propylene.



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propylene. The break into two C3 radicals must be less easy

than this into a C2 a n d a C4, explaining the higher

concentration of C2 products than this of propylene from

the hexene fuel [8]. From hexane (Fig. 1). Hexane looses an

H (preferably secondary [13]) to form a C6 radical which

can give a C3 radical and a propylene. As previously, the

first radical can give propylene after an H extraction. As in

the case of 1-hexene, hexane can preferably give a C2 and a

C4 radical instead of two C3 ones [15], explaining the higher

concentration of C2 products than propylene obtained from

the hexane fuel. From cyclohexane (Fig. 3). Cyclohexaneopens the ring and the C6 radical formed follows the

mechanism described in the case of 1-hexene. As this

mechanism has a supplementary stage, the participation of

cyclohexane to the formation of propylene is lower than this

of 1-hexene. From n-octane (Fig. 6). Octane gives

preferably a C2 and a C6 radical after a b-scission. The C6one can follow the previous mechanism to give propylene.

This mechanism has one more stage for the formation of

propylene than this of 1-hexane explaining the lower

participation of octane than 1-hexene to the formation of

this pollutant.

Isobutane. The formation paths of isobutane from

isooctane are presented in literature ([16,17], Fig. 5).

According to these articles, isooctane gives an isooctylradical followed by two C4 parts. Both radicals can give an

isobutane after combination with an H. This reaction takes

place rather at the end of the reaction before the cooling

phase; if oxidation continues, C4 radicals will give lighter

products. Another path is the formation of a tertiary isooctyl

radical [13], which is decomposed to isobutyl radical and

isobutene following a b-scission. The isobutyl radical can

give isobutene with a combination with H. The first path can

be applied in the case of MTBE (Fig. 7).

1-Butene: from n-octane (Fig. 6). n-Octane breaks in two

C4 parts, a C4 radical and isobutene [18]. The first C4 radical

can also give a 1-butene after an H extraction [12]. From n-

hexane (Fig. 1). Hexane forms initially a secondary radical

in position 2 or 3 [15]. The 3-hexyl radical gives 1-butene or

a C4H7 radical that can give 1-butene after an H extraction,

while the 2-hexyl breaks into two C3 products [19]. From

1-hexene (Fig. 2). 1-Hexene, looses an H in allylic position

and forms the 3-hexenyl radical which is quite stable for

resonance reasons [12,19]. This radical gives, by b-scission,

a C4H7 radical that takes an H to form 1-butene, or looses an

H to form 1,3-butadiene [12]. An attack of O or OH on

the 1-hexene can directly give a C4H7 radical and an

ethylene or a 1-butene and a C2H3 radical [19]. Another path

is the addition of an H to 1-hexene to form a C6H13 that

Fig. 5. Proposed path for the isooctane combustion and formation of

propylene, isobutene, isobutene and 1-butene.

Fig. 6. Proposed path for the octane combustion and formation of ethylene,

propylene and 1-hexene.



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gives an ethylene and a primary C4H9 radical [19]. This last

looses an H to form 1-butene [12]. From isooctane (Fig. 5).

This path must be quite complex; isooctane gives initially an

isooctyl radical that breaks, giving principally isobutene and

an isobutyl radical [15]. A methyl radical must migrate

before the formation of 1-butene. The complexity of this

path explains the low contribution of isooctane to theformation of this pollutant.

Isobutene: from isooctane (Fig. 5). Isooctane produces

preferably an tertiary isooctyl radical, but it can also be a

secondary or primary one [13]. The tertiary radical is

decomposed to isobutyl radical and isobutene following a

b-scission. The tertiobutyl radical is preferably decomposed

to a methyl radical and propylene [14], but it can also give

an isobutene or isobutane loosing or taking an H [12]. The

secondary radical mainly gives a methyl and 2,4-dimethyl-

pentene or 2,2-dimethylpentene [20]. The primary iC8radical is decomposed in the same place as the tertiary

giving the same products (isobutene and tertiobutyl); it can

also give propylene and a neo-pentylene radical. This last is

decomposed in methyl and isobutene [14]. At very high

temperature, isooctane can give directly the tC4H9 and

iC4H9 radicals, or neoC5H11 and iC3H7, which continue by

the previous reactions [13]. From MTBE (Fig. 7). MTBE

looses initially an H [14,21]. The CH bond of the methyl is

weaker than this of tertiobutyl [22]. This radical breaks into

two smaller pieces, a tertiobutyl radical and formaldehyde,

or isobutene and a CH3O radical [22]. The (CH3)3C radical

can give isobutene after an H extraction. Another path is the

direct decomposition of MTBE to give isobutene and

methanol [22].

Cis and trans-2-butene. Our results are not sufficient to

propose a path for their formation. o-Xylene must break to

two C4 radicals with a breaking point between the two

methyls. These radicals must then react with H to form the

cis or trans form of 2-butene.

Butadiene: from cyclohexane (Fig 3). Cyclohexane

looses an H to form a C6H11 radical, which opens to givea straight chain radical [23]. This last one can react with an

H to form 1 hexene or break into two parts by a b-scission

and form 1,3-butadiene [19,23]. From 1-hexene (Fig. 2). A

direct attack of O or OH on the 1-hexene can give ethylene

and a C4H7 radical [19]. This last one can loose an H and

form 1,3-butadiene. Another probable path is the formation

of a C6H11 after an H extraction, a b-scission and formation

of 1,3-butadiene after an H extraction. Hexane does not

produce 1,3-butadiene, because its initial radical is pre-

ferably secondary and it is decomposed by a b-scission to

give smaller products. To obtain 1,3-butadiene from hexane,

the following reaction must take place: formation of a

primary radical, then formation of 1-hexene from this

radical or directly from hexane, which must loose an H at

the opposite side of the double bond and continue with the

reactions presented previously. However, as hexane does

not produce 1-hexene, this path does not take place.

Isopentane. The formation mechanism of isopentane

from isooctane must be based on the consecutive extraction

of three methyl radicals from this fuel component and the

combination with an H for the termination reactions, or the

re-arrangement of an isopentyl radical.

1-Hexene. A part of exhaust 1-hexene comes from the

unburned fuel; the rest comes from cyclohexane (Fig. 3) and

n-octane (Fig.6). As in thecase of 1,3-butadiene,cyclohexaneforms a C6 straight chain radical [23], which forms 1-hexene

after a combination with an H. It must be noticed that hexane

does not produce 1-hexene, because it forms preferentially

secondary radicals, which are broken into smaller pieces.

n-Octane gives a and a C6 radical after a b-scission. This

radical gives 1-hexene after an H extraction [24].

Cyclohexane. The majority of cyclohexane comes from

unburned fuel; its formation from aromatics proceeds by a

dealkylation followed by a hydrogenation (Fig. 8).

Benzene (Fig. 8). Fuel benzene gives the majority of

exhaust benzene. Fuel aromatics proceed by a dealkylation;

the benzyl radical formed is then combined with a hydrogen

to give benzene. This reaction is more important at rich

conditions due to the lack of oxygen. At lean ones, the

intermediate products are further oxidized to CO. The

detailed mechanism of benzenes formation from higher

aromatics is presented in literature [20,25]. Benzene is

formed from cyclohexane by the consequentially loss of

three atoms of hydrogen. A double bond must be formed

first. Several mechanisms can produce it, as for example:

oxidation to alcohol and abstraction of a molecule of water

[25] or production of an alkyl radical by abstraction of a first

and then a second H [18]. Following these mechanisms, the

second and the third double bond are then formed to give

Fig. 7. Proposed path for the MTBE combustion and formation of

isobutene, formaldehyde and methanol and probable formation of

isobutane.



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benzene. The formation of benzene from cyclohexane is

more important at rich than at lean conditions, but its

specific weight is more important at lean ones. Another

reaction for benzenes formation is the addition of propargyl

radicals to allene [24]. As many fuel compounds can give

these two radicals, benzene can be formed from almost

every fuel. But, as the exhaust concentrations measured in

the case of fuels without aromatics or cyclohexane are very

low, this last reaction is very minor comparing to the direct

formation from aromatics. The formation of benzene from

isooctane follows probably this last path.

Toluene (Fig. 8). The majority of exhaust toluene comes

from the unburned fuel. Its formation from o-xylene followsan extraction of the methyl radical and a combination with

an hydrogen to give toluene, or by the formation of

intermediate oxygenated products. These detailed mechan-

isms are presented in literature [20]. ETB forms an initial

radical in a position [25,26]. The next step can be:

FCH2CH30FCH2CCH3, or FCH2CH30FCHCH3CH,

or FCH2CH30FCCH2CH3. The enthalpies of these

reactions are 306, 331 and 376 KJ/mole, respectively [27].

These data indicate that the most favorable path is the first

one; the combination of an H can then give toluene. In both

cases, o-xylene and ETB, the formation of toluene needs

the combination of an H. This must be a termination

reaction that takes place during the cooling phase.

ETB (Fig. 8). A part of exhaust ETB comes from the

unburned fuel. Toluene loses an H to form a F-CH2, which

can react with a methyl to give ETB. o-Xylene loses first a

CH3; the formed radical must then give a F-CH2 by internal

arrangements, which reacts with a methyl to give ETB. Asduring the combustion process, the reactions give generally

smaller products, these last two reactions must take place

during the cooling phase.

o-Xylene (Fig. 8). Contrary to ETB, o-xylene is not

coming from other fuel aromatics, but it is only a product

of the unburned fuel. The only probable path to this

formation could be the extraction of a hydrogen from the

aromatic nucleus of toluene, and the addition of a methyl

radical at this place. This mechanism is quite unlike to

happen even during the cooling phase; this aromatic gives

rather lower products. The same remarks are valuable in

the case of ETB.

Isopropylbenzene (Fig. 8). ETB firstly loses an H. The

radical formed can follow two ways: the transformation to

lower intermediate products (toluene, benzene), or the

reaction with CH3 to form iPB. This reaction needs the

formation of C6H5CHCH3, which is less enhanced than that

of C6H5CH2 [25]; for this reason the principal product of

ETB is toluene. The combination of a methyl radical

probably takes place during the cooling phase. In the case of

o-xylene, this one loses initially a CH3. This radical is then

transformed to a F-CH2 by internal arrangements, as for the

formation of ETB, and follows the second path of the

previous reaction. Toluene loses initially a hydrogen to form

the F-CH2 which follows also the previous reactions. Thecontribution ofo-xylene is lower than this of ETB due to the

more complicated path needed for the formation of iPB.

Toluene gives less iPB because it gives preferably benzene

or lower products.

Pentane, hexane, octane and isooctane are products only

of the unburned fuel.

4. Conclusions

The previous article [8] work allowed the qualitative

determination of the precursors of each exhaust specifichydrocarbon. This work allowed the quantitative determi-

nation of each precursor at stoichiometry, lean and rich

conditions.

Concerning the emissions of main exhaust hydrocarbons

at stoichiometry:

the majority of ethylene comes from fuel octane (25.3%),

hexane (19.6%), 1-hexene (18.2%) and cyclohexane

(15.3%), while the majority (80%) of acetylene comes

from fuel benzene,

the most important source of propylene at stoichiometry

is fuel isooctane (24.1%), 2-propanol (23.3%), 1-hexene

Fig. 8. Proposed path for the combustion of aromatics and formation of

ethylene, cyclohexane, benzene, toluene, ETB, o-xylene and iPB.



9/9

(20.0%), octane (15.6%), hexane (13.3%) and cyclohex-

ane (3.7%),

1-butene is formed from fuel octane (36.3%), hexane

(32.8%), hexene (21.2%) and isooctane (9.6%); most of

the half (55.6%) of exhaust isobutene comes from fuel

MTBE and the rest from isooctane; more than the half of

exhaust 1,3-butadiene comes from fuel cyclohexane(50.5%); the other sources are 1-hexene (19.7%), o-

xylene (11.3%), octane (10.7%), isooctane (4.0%) and

hexane (3.8%),

isopentane is mainly produced by fuel isopentane, but

small amounts (11.7%) come from fuel isooctane,

the majority (71%) of exhaust 1-hexene comes from fuel

1-hexene, 27% from fuel cyclohexane, but a small

quantity (2%) from fuel octane; the majority (80%) of

exhaust cyclohexane comes from fuel cyclohexane, but

aromatics contribute with 36% each? the majority

(82%) of exhaust benzene comes from fuel benzene; the

other sources are ETB, toluene, o-xylene andcyclohexane,

the two-thirds of toluene come from fuel toluene,

while fuel o-xylene contributes with 26% and the rest

comes from fuel ETB; exhaust ETB is mainly (85%)

produced from fuel ETB, while the rest comes from

toluene and o-xylene; the majority (81%) of isopro-

pylbenzene comes from fuel ETB; fuel o-xylene

contributes by 15% and the rest 4% comes from

fuel toluene,

no model is found in the case of methane and ethane,

because these HC have many precursors, not always

related with the initial fuel composition, n-pentane, hexane, octane, isooctane and o-xylene come

only from the respective fuel components,

for each exhaust hydrocarbon, a detailed formation path

is proposed.

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