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Emission of Alcohols and CarbonylCompounds from a Spark IgnitionEngine. Influence of Fuel andAir/Fuel Equivalence Ratio

E . Z E R V A S , * , X . M O N T A G N E , A N DJ . L A H A Y E

Institut Francais du Petrole, 1 et 4 avenue du Bois Preau,F-92500 Rueil-Malmaison Cedex, France, and Institut deChimie des Surfaces et Interfaces, 15 rue Jean Starcky,F-68057 Mulhouse Cedex, France

A spark ignition engine was used to study the impact offuel composition and of the air/fuel equivalence () ratio onexhaust emissions of alcohols and aldehydes/ketones.Fuel blends contained eight hydrocarbons (n-hexane,1-hexene, cyclohexane,n-octane, isooctane,toluene,

o-xylene, and ethylbenzene (ETB)) and four oxygenatedcompounds (methanol, ethanol, 2-propanol, and methyl tertbutyl ether (MTBE)). Exhaust methanol is principallyproduced from fuel methanol and MTBE but also fromethanol,2-propanol, isooctane,andhexane.Exhaust ethanoland 2-propanol are produced only from the respectivefuel compounds. Exhaust formaldehyde is mainly producedfrom fuel methanol, acetaldehyde from fuel ethanol, andpropionaldehyde fromstraight-chain hydrocarbons. Exhaustacroleine comes from fuel 1-hexene, acetone from2-propanol, n-hexane, n-octane, isooctane, and MTBE.Exhaust crotonaldehyde comes from fuel 1-hexene,cyclohexane, n-hexane, and n-octane, methacroleine fromfuel isooctane, and benzaldehyde from fuel aromatics.

Light pollutants (C1-C2) are most likely formed fromintermediate species which are quite independent of thefuel composition. An increase in increases the exhaustconcentration of acroleine, crotonaldehyde, methacroleine,and decreases these of the three alcohols for the alcohol-blended fuels. The concentration of methanol, formal-dehyde, propionaldehyde, and benzaldehyde is a maximumatstoichiometry.Theexhaustconcentrationof acetaldehydeand acetone presents a complex behavior: it increasesin some cases, decreases in others, or presents a maximumat stoichiometry. The concentration of four aldehydes(formaldehyde, acetaldehyde, propionaldehyde, andbenzaldehyde) is also linked with the exhaust temperatureand fuel H/C ratio.

In t roduct ionCorrelations between fuel composition and exhaust emis-sions from spark ignition (SI) engines operating at stoichio-metric conditions have been well-established in the case ofregulated pollutants (refs 1-4and many others). However,

exhaust gascontainsotherspecific pollutantssuch as alcoholsand carbonyl compounds that have, thus far, not beenthoroughly investigated. Moreover, modern SI engines arenot working only under stoichiometry but also under leanand rich conditions. The influence of air/fuel equivalenceratio () on the emission of specific pollutants is not verywell-established.

Carbonyl compounds are emitted from internal combus-tion engines as products of incomplete combustion ofhydrocarbons or oxygenated compounds. They are finallyfound in the atmosphere, and many authors studied theirdistribution andreactions in atmosphere of urban andruralareas (refs 5-9 and many others). These pollutants havemultiple sources, but motor exhaust gas is considered asone of the most important (9). The fuel composition caninfluence theemissions of these pollutants (2, 4, 10). Gasolinecontains basically hydrocarbons, but alcohols or methyl tertbutylether (MTBE) arealso added in fuels to decreaseexhausthydrocarbons and carbon monoxide. The addition of thesecompounds increases the emission of some oxygenatedpollutants, as methanol from fuel containing methanol orMTBE(11, 12), formaldehyde from fuel containingmethanol(11), or acetone andacroleine from this containing MTBE (4,10). No study reports the influence of on the emission ofalcohols and carbonyl compounds.

In a previous work (13), we presented the influence offueland of on theemission of organic acids of an SI engine.The main conclusions were that the first four aliphatic acidsareenhanced fromfuelaromatic andoxygenated compoundsand are strongly depended on . Continuing this work, thispaper presents the influence of fuel and on the emissionof alcohols andcarbonylcompounds from an SI engine. Theinfluence of the fuel was studied by the use of specific fuelblends containing hydrocarbons and oxygenated com-pounds. Relationsbetween exhaust alcohols and aldehydes/ketones and fuel parameters or other exhaust compoundswere also researched. On the basis of the obtained results,some likely formation paths of these compounds are

proposed.

Experimental SectionEngine and Operating Conditions. A Cooperative FuelResearchCommittee (CFR) SI engine wasused for these tests.This engine is a small monocylinder engine used for octanenumber determination. The used was from 0.83 to 1.25(calculated from the exhaust gas analysis), while all otherengine parameters were kept constant (13, 14). Modernengines emit less pollutants than the CFR engine, but thisengine allows for the determination of the most importantcorrelations between fuel composition and pollutants emit-ted. As the subject of this work was to find out the afore-mentionedcorrelations, no catalytic converterwas used. Ourexperience shows that tailpipe emissions of the pollutants

presented in this paper (except probably formaldehyde andacetaldehyde) are lower than the engine out ones becauseof partial oxidation on the catalytic converter.

FuelsUsed. Twofuelmatrixeswere adopted in this study.The first one (synthetic fuels matrix) contains eight hydro-carbons (n-hexane, 1-hexene, cyclohexane, n-octane, iso-octane(2,2,4-trimethylpentane),toluene, o-xylene, and ethyl-benzene (ETB)), while the second one (oxygenated fuelsmatrix) alsocontains four oxygenatedcompounds (methanol,ethanol, 2-propanol, and MTBE). An experimental designwas used to determine the quantity of each component inthe blend of synthetic fuels. To avoid a high dispersion inphysical properties of the fuels used, an alkylate (containing

* Corresponding author phone: 331-69 27 84 77; fax: 331-69 2781 40; e-mail: [email protected]. Present address:Renault-CTLL26146, 1, Allee Cornuel, Fr-91510 Lardy, France.

Institut Francais du Petrole. Institut de Chimie des Surfaces et Interfaces.

Environ. Sci. Technol. 2002, 36, 2414-2421

2414 9 ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 36, NO. 11, 2002 10.1021/es010265t CCC: $22.00 2002 American Chemical SocietyPublished on Web 04/23/2002



basically isooctane but also 1.5% of benzene) was used asthebasefuel fortheseblends.The referencefuel(R) containsan equal content of each of the eight compounds, while theother fuels contain 42% of a major component. The oxygen-

ated matrix was obtained by the addition of 5% or 20% ofone of the four oxygenated compounds to the fuel R. Twoother fuels were also used. The first fuel is pure isooctane(iC8); the second is a mixture of 80% 2,2,4-trimethylpentaneand 20% toluene (iC8T). These fuels allow for the study ofthe addition of an aromatic component to an alkylate basis.Finally, a commercial gasoline was also tested.

Moredetailsabout these fuels (chemical composition andoctane number) are presented in Table 1. The distillationproperties of these fuels are presented elsewhere (14). Thechemical composition andphysicalproperties of these fuelsare quite different from the commercial one, but thesematrixes allow for the study of the influence of the chemicalcomposition of the fuel on the pollutants emission.

Analysis of Exhaust Gas. Alcohols were collected by

passinga sample of rawexhaustgas broughtthrough a heatedline through two impingers in series containing 20 mL eachof deionized water.The gasflow rate wasregulatedat 1 L/min.Thefinal solutionwas analyzedby gaschromatographyusingflame ionization detector (GC/FID) following the proceduredescribed by Williams et al. (11) and Siegel et al. (15). Astandard solutioncontaining six alcohols(methanol, ethanol,n-propanol, 2-propanol, n-butanol, and2-butanol) was usedfor chromatograph calibration and for the identification ofeach alcohol. The detection limits of this method are under0.5 ppm in the solution (14). Only methanol, ethanol, and2-propanol were found in the exhaust gas and were well-separated under these analytical conditions.

Carbonyl compounds were collected in 2 30 mL of anacidified 2,4-dinitrophenylhydrazine (DNPH) solution inacetonitrile. The gas flow rate was regulated at 1 L/min. Thefinal solution was analyzed by high performance liquidchromatography using ultraviolet detection (HPLC/UV)following the procedure described by Lipari and Swarin (16)and Swarin et al. (17). A standard solution containing 13aldehydes (formaldehyde, acetaldehyde, acroleine, acetone,propionaldehyde, crotonaldehyde, methacroleine, methylethyl ketone (MEK), butyraldehyde, benzaldehyde, and o-,m-,and p-toluenealdehyde)was usedfor thechromatographcalibration and for the identification of each aldehyde. Thedetection limits of this method were under 5 ppb in thesolution. All of these carbonyl compounds were detected inthe exhaust gas and had no interference with the otherexhaust gas components, but the column used cannot

separate MEK and n-butyraldehyde. The three toluenalde-hydes were detected in very low concentrations, so they arenot presented here. Organic acids and alcohols do not reactwith the DNPH solution (18). More details are presented

elsewhere (14).Repeatability Tests.At each, five identical points of the

fuel R were used to evaluate the repeatability of the engineand the analytical methods. The relative standard deviationof the concentration of most carbonyl compounds andmethanol (fuel R does not emit other alcohols) was found tobe less than 15%. Alltests were repeated,and average valueswere used.

Results and Discussion

Percentage of Alcohols and Carbonyl Compounds in theOther Exhaust Pollutants. The quantity of each aldehyde/ketone, as a percentage of the total carbonyl compoundsemitted, is presented in Table 2 for the commercial fuel atstoichiometry (calculated on a C1 basis). The major carbonyl

compound emitted is benzaldehyde (due to fuel aromatics)followedby formaldehydeand acetaldehyde. Concerning theother fuels at ) 1.0, these percentages vary from 25% to47% for the formaldehyde, 6-30% for the acetaldehyde, and1-50% for the benzaldehyde. Concerning alcohols, the E5and E20 fuels emit 71% and 91% of ethanol and the P5 andP20 fuels 72% and 92% of 2-propanol, respectively; the restis methanol. All other fuels emit only methanol.

The emission range of total and individual alcohol,carbonyl compounds, and total organic acids and HC arealso presented in Table 2. In the case of the commercial fuelatstoichiometry, methanol corresponds to 0.02% of thetotalHC and carbonyl compounds to 3.5%. Formaldehyde cor-responds to 1.1% of the total HC, acetaldehyde to 0.3%, andbenzaldehyde to 1.2%. The higher percentage of methanol

is 0.6% in the case of the M20 fuel. Carbonyl compoundsvary from 2.3% (xylene fuel) to 6% (E20 fuel) of total HCemitted.

The influences these ratios (Table 2). In the case of thecommercial fuel, total HC decreases with, while methanol,total aldehydes, andorganic acids present a maximum valueat stoichiometry. These values indicate that a greaterimportant percentage of exhaust pollutants are found inoxygenated form under lean conditions (7% at ) 0.83, 27%at ) 1.0, and 55% at ) 1.25).

Methanol. Figure 1 presents that, at stoichiometry,exhaust methanol is principally emitted from themethanol-blended fuels but also from MTBE (13). Fuel ethanol and

TABLE 1. Chemical Analysis and Octane Number of the Synthetic Fuels Used a

synthetic fuels

oxygenatedfuels

hexane hexenecyclo-

hexane octaneiso-

octane toluene xylene ETB R M5 E5 P5 MTBE5 M20 E20 P20 MTBE20

hexane 42 2 2 2 2 2 2 2 7 6.65 6.65 6.65 6.65 5.6 5.6 5.6 5.61-hexene 2 42 2 2 2 2 2 2 7 6.65 6.65 6.65 6.65 5.6 5.6 5.6 5.6cyclohexane 2 2 42 2 2 2 2 2 7 6.65 6.65 6.65 6.65 5.6 5.6 5.6 5.6n-octane 2 2 2 42 2 2 2 2 7 6.65 6.65 6.65 6.65 5.6 5.6 5.6 5.6isooctane 2 2 2 2 42 2 2 2 7 6.65 6.65 6.65 6.65 5.6 5.6 5.6 5.6toluene 2 2 2 2 2 42 2 2 7 6.65 6.65 6.65 6.65 5.6 5.6 5.6 5.6o-xylene 2 2 2 2 2 2 42 2 7 6.65 6.65 6.65 6.65 5.6 5.6 5.6 5.6ETB 2 2 2 2 2 2 2 42 7 6.65 6.65 6.65 6.65 5.6 5.6 5.6 5.6methanol 5 0 0 0 20 0 0 0ethanol 0 5 0 0 0 20 0 02-propanol 0 0 5 0 0 0 20 0MTBE 0 0 0 5 0 0 0 20oxygen (% wt) 3.08 1.98 1.45 0.90 11.89 7.75 5.72 3.60alkylate 44 44 44 44 44 44 44 44 44 41.8 41.8 41.8 41.8 35.2 35.2 35.2 35.2RON 63.7 83.7 88.7 43.6 93.8 101.3 96.6 100 85.2 89.4 90.4 87.5 86.8 98.6 97.4 96.6 93.1

a % vol contents.

VOL. 36, NO. 11, 2002 / ENVIRONMENTAL SCIENCE & TECHNOLOGY 9 2415



2-propanol also contribute to the emission of exhaustmethanol as the E5, E20, P5, and P20 fuels emit moremethanolcomparingto fuel R.Figure 1 shows that methanolis also produced from two hydrocarbons: hexane andisooctane. A comparison between iC8 and iC8T fuels shows

that toluene does not participate in the formation ofmethanol; the second fuel emits 80% of the first one.

Commercial fuel produces about the same quantity ofmethanol as fuel R.

Exhaust methanol can come from the unburned fuelmethanol or from a recombination of a CH3 and OH radicalor a CH3O and H (free or abstraction of an H from anhydrocarbon). Following the first reaction path, isooctaneenhances the formation of methanol probably because ofthe higher amountof CH3 radicals formed, confirmedby thehigher amounts of methane emitted from pure isooctanethanfrom isooctane/toluene(18). However, other fuels (suchas octane or xylene)also emithighermethane concentrationsthanthe R fuelwithoutenhancing the formationof methanol(18). Hexane enhances the methanol formation but emitslower methaneconcentration thanthe fuelR (18). Our resultscannot prove the second possible path (CH3O + H), as no

relation between methanol emissions and formaldehydeemissions is found. The third reaction path (CH3O + RH wCH3OH + R) probably takes also place.

The exhaust concentration of methanol is maximum atstoichiometry forallfuelsusedexcept forM5 and M20, whichincrease exhaust methanol at rich conditions due to un-burned fuel (Figure 1). In the case of all other fuels, twohypotheses can be proposed as in the case of organic acidsemissions (13): the decrease of exhaust methanol concen-tration at rich conditions must be due to the preferableformation of CO, and at lean conditions, the precursors ofmethanol must rapidly oxidized to other products.

A quantitative model relating the exhaust concentrationof methanol with the percentages (in volume) of the fuelcomponents is thefollowing (all models presentedhere take

also into account the composition of alkylate)

The r2 of the line ,predicted values. ) a1 ,experi-mental values. + b1 is 0.981, with a1 ) 1.0012 and b1 )0.0697, indicating a verygood accordance between predictedand experimental values. According to this model, at stoi-chiometry and for equal fuel components content, themajority (51.9%) of the exhaust methanol comes from the

TABLE 2. Percentage of Each Carbonyl Compound over Total Carbonyl Compounds (as C1) , Range of the Emiss ions of Alcoholsand Carbonyl Compounds Detected (for Al l Fuel and Used, in ppmv), Emission of Total Alcohols, Carbonyl Compounds, OrganicAcids and HC for the Commerc ial Fuel for Three (1.25, 1.0, and 0.8, as C1) , and Percentage of These Totals over the Total HCEmit ted for the Commerc ial Fuel at Stoichiometry

%carbonyl/total carbonyls(fuel ) COM, ) 1.0,

inC1 basis)

range,all fuels, ) 1.0

(ppmvina C1 basis)

%overtotal HC,(COM, ) 1.0,

inaC1 basis)

COM, ) 1.25,

(ppmv)

COM, ) 1.0,(ppmv)

COM, ) 0.83,

(ppmv)

alcohols 0.15-3.6 0.02 0.07 0.21 0.06total carbonyls 21-42 3.5 41 42 28total organic acids 35-291 24 244 276 150

total HC 457-1131 2100 880 230methanol 0.15-3.6 0.02ethanol 0-0.302-propanol 0-3.0formaldehyde 31.5 8.6-16.4 1.1acetaldehyde 9.4 2.0-11.4 0.3propionaldehyde 1.7 0.45-0.90 0.07acetone 6.1 1.44-11.4 0.23acroleine 8.9 1.98-4.14 0.34crotonaldehyde 1.9 0.44-0.92 0.07methacroleine 2.1 0.58-2.68 0.08MEC + n-butyraldehyde 2.4 0.64-1.12 0.09benzaldehyde 36 0.35-18.2 1.2

FIGURE 1. (Bottomgraph)Fuel effectonformaldehyde emissionsatstoichiometry: (mean)meanexhaustconcentration;(min,max)minimumandmaximumconcentrationfora95%confidence level.(Middle curves) effect on methanol emissions for some repre-sentativefuels. (Uppercurves) effectonethanol and2-propanolemissions for the E5, E20, P5, and P20fuels.

exhaust methanol (ppmv) ) a fuel methanol (%) +b fuel MTBE (%) + c fuel isooctane (%) +d fuel hexane (%) + e fuel ethanol (%) +

f fuel 2-propanol (%) + g.

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fuel methanol, 5.9% from MTBE, less than 2% each fromethanol, isooctane, 2-propanol, and hexane, and the rest(40.5%) from all other fuel components (Table 3). This lastpercentage is quite important andindicates that, as methanolis a light compound, its exhaust concentration can comefrom all fuel components. The percentage of the fouroxygenated compounds increases at rich conditions due tounburned fuel and to their cracking to methanol, while thisof the other fuel components decreases (Table 3).

Ethanol and 2-Propanol. Ethanol and 2-propanol are

products only of the unburned fuel (they are detected onlyin the case of E5, E20, P5, and P20 fuels). The absence ofthese alcohols in theexhaust gasof other fuels indicates thatthe CH3CH2 and (CH3)2CH or CH3CH2O and (CH3)2CHOradicals formedduring the combustionprocess cannot reactwith OH or H to give ethanol and 2-propanol, respectively,butgive smallercompounds or aldehydes.As exhaustethanoland 2-propanol come only from the unburned fuel, theirconcentration increases in rich conditions (Figure 1). Themodels exhaust ethanol ) a fuel ethanol and exhaust2-propanol ) a fuel 2-propanol can be used. The r2 of thelines ,predicted values. ) a1,experimental values. +b1 is 0.99 for both compounds, with a1 ) 1.004 and 0.999and b1 ) -0.096 and 0.009, respectively, indicating avery good accordance between predicted and experimentalvalues.

Formaldehyde. Literature reports that exhaust formal-dehydeis produced from fuel methanol,ethanol,and MTBE(12, 19, 20) and also by the decrease of fuel aromatics (4)because these compounds do notparticipate in itsformation.Our results are in accordance with those reported; Figure 2shows that fuels containing 42% of aromatics produce lessHCHO than fuel R andthatiC8T fuel produces 80%of HCHOcomparing to iC8. Formaldehyde is mainly produced fromoctane, isooctane, methanol, and MTBE. At stoichiometry,the addition of methanol to fuel R increases exhaust HCHOby40-80%.The other three oxygenated compounds enhancethe formation of formaldehyde to a lesser extent (10-40%),with little difference between low and high content of

TABLE 3. Part ic ipat ion of t he Di fferent Fuel Components i n the Emiss ions of Exhaust Pol lutantsa

methanol hexane isooctane methanol ethanol 2-propanol MTBE constant

) 1.25 0.58 4.06 25.91 0.41 0.36 25.91 66.46 ) 1.11 0.32 2.92 29.15 0.92 0.72 29.15 63.38 ) 1.0 0.11 1.18 51.85 1.56 1.15 51.85 40.93 ) 0.91 0.06 0.34 78.35 2.70 1.87 78.35 12.02 ) 0.83 0.04 0.25 82.72 2.97 2.04 82.71 8.03

acetaldehyde hexane 1-hexene n-octane ethanol 2-propanol

) 1.25 0.42 0.56 0.50 6.17 1.65

) 1.11 0.88 0.74 0.99 7.23 1.65 ) 1.0 1.10 1.13 1.55 13.72 3.21 ) 0.91 0.97 0.75 1.69 21.23 4.27 ) 0.83 0.99 0.48 1.39 29.41 3.47

acetone 1-hexene benzene toluene 2-propanol MTBE constant

) 1.25 0 0 3.21 10.93 0.24 85.61 ) 1.11 0 0 2.22 13.94 0.51 83.30 ) 1.0 1.50 0.86 1.21 15.19 3.67 77.54 ) 0.91 0.49 0.51 0.93 21.82 2.32 73.90 ) 0.83 0.24 0.22 0.96 24.95 1.91 71.69

benzaldehyde benzene toluene ETB xylene

) 1.25 47.54 28.04 19.60 4.82 ) 1.11 26.35 36.99 28.73 7.93 ) 1.0 6.10 42.6 32.37 18.93

) 0.91 42.85 24.85 21.95 10.36 ) 0.83 46.74 23.85 20.60 8.81

a Percentages of this participation for the five different (pollutants in ppmv, fuel components in %, constant ) all other fuel components).

FIGURE 2. (Bottomgraph)Fuel effectonformaldehyde emissionsatstoichiometry: (mean)meanexhaustconcentration;(min, max)minimumandmaximumconcentrationfora95%confidence level.(Middle) Fuel effect onacetaldehydeemissions atstoichiometry.(Upper curves) effect for some representativefuels.




oxygenated compounds. Commercial fuel produces aboutthe same quantity of formaldehyde as does the R fuel.

The formation mechanism of HCHO from methanol andMTBE is already known (21-23). The formation of HCHOfromisooctane mustfollowthe sameinitialpath as methanol(excess of CH3 radicals). As formaldehyde is the lighteraldehyde, it musthave multiplesourcesnot always correlatedwith initial fuel composition.

The exhaust concentration of formaldehyde is maximumat stoichiometry for all fuels tested (Figure 2). The sameconclusions as for methanol may be presented here: the

decrease of exhaust formaldehyde at rich conditions mustbe due to the preferable formation of CO, while, at leanconditions, the precursors of formaldehyde must rapidlyoxidized to other products.

As exhaust formaldehyde has multiplesources, not alwayslinked with initial fuel composition, no model correlating itsexhaust concentration with the fuel composition has beenfound.

Acetaldehyde. Literature presents that fuel ethanol in-creases acetaldehyde, that MTBE has no effect (14, 19, 24),and that a decrease of fuel aromatics increases its emission(4). Our results are in accordance with those presented:exhaust acetaldehyde is principally produced from the fuelethanol (Figure 2). Toluene and xylene decrease exhaustacetaldehyde as compared to fuel R, and the iC8T fuel

produces less (85-90%) acetaldehyde than the iC8 fuel.Exhaust acetaldehyde is also produced from the straight-chain hydrocarbons n-hexane, 1-hexene, and n-octane.Methanol and MTBE do not significantly influence itsemission, but 2-propanol enhances it, especially at highcontent (P20). Commercial fuel produces less exhaustacetaldehyde than fuel R due to higher aromatic content.The addition of ethanol to fuel R increases exhaust acet-aldehyde by 80-280%, of 2-propanol by 20-70%, while thechanges from theaddition of methanol andMTBEare within-15 to 15%.

The formation mechanism of acetaldehyde is alreadyknown (21, 22, 25). Straight-chain hydrocarbons enhancethe formation of acetaldehyde by the C2 radicals producedfrom scissions.

Acetaldehyde concentration increases at rich conditionsin the case of ethanol-blended fuels (due to oxidation ofethanol to acetaldehyde). It presents a maximum at stoi-chiometry in the case of fuels containing the compoundsthat enhance its formation (n-hexane, 1-hexene, n-octane,and 2-propanol). The reason is that, in lean conditions,acetaldehyde precursors are rapidly oxidized, and in richconditions, they are preferably transformed to CO than toacetaldehyde due to lack of oxygen. The exhaust concentra-tion of acetaldehydeincreases atlean conditions forall otherfuels. The influence of on the emission of acetic acid isquite similar (13).

A model can correlate the exhaust concentration ofacetaldehyde with the content of fuel ethanol, n-hexane,1-hexene, octane, and 2-propanol

The r2 of the line ,predicted falues. ) a1 ,experi-mental values. + b1 is 0.98, with a1 ) 0.99 and b1 ) 0.013,indicating a very good accordance between predicted andexperimental values. According to this model, at stoichi-ometry and for equal content of all fuel components, the13.7% of the exhaust acetaldehyde comes from fuel ethanol,3% from fuel 2-propanol, and 1-1.7% each from hexane,1-hexene, and octane (Table 3). The other 79% comes fromall the other fuel components (from the C2 radicals which

are further oxidized to acetaldehyde). The specific weight ofethanol and 2-propanol increases at rich conditions whilethis of the three hydrocarbons presents a maximum valueatstoichiometryandthis of theothersources decreases(Table3). In the case of the fuel R, the addition of ethanol increaseslinearly the exhaust acetaldehyde: exhaust acetaldehyde )a fuel ethanol + b(with a) 0.208 and b) 1.6 at ) 1.0).The exhaust concentrations of these two compounds arealso linked (Figure 3). The line exhaust concentration ofacetaldehyde) 2.51 exhaust concentration of ethanol + 1.47indicates that the majority of fuel ethanol is oxidized toacetaldehyde and that only 1/2.5 is emitted as ethanol.

Propionaldehyde. Exhaust propionaldehyde is producedfrom the straight-chainhydrocarbons(hexane,1-hexene, and

octane, Figure 4), and fuel aromatics decrease its exhaustconcentration.The addition of oxygenated compounds doesnot influence exhaust propionaldehyde. However, as nodecrease of this concentration occurs(becauseof thedecreasethecontent of thestraight-chainhydrocarbons),it is probablythat oxygenated compounds facilitate the oxidation of thepropionaldehyde precursors. Fuel iC8 produces a slightlyhigher concentration of propionaldehyde than fuel iC8T,indicating that isooctane can generate slightly more easilythan toluene a C3 radical that forms propionaldehyde.Commercial fuel produces less propionaldehydethan fuel Rdue to higher content of aromatics.

The formation reaction of this aldehyde needs a propylradical(21). Straight-chain hydrocarbonscan givethis radicalby the following reactions: hexane wCH3(CH2)3CHCH3 wCH3CH2CH2 + CH2dCHCH3 (26). 1-Hexene preferably pro-duces the CH3(CH2)2CHCHdCH2 (26), but this radical isfurther broken to CH2dCH2 and CH2CH2CHdCH2. Theformationof a CH3CH2CH2 radical needsthe followingdirectdissociationreaction: 1-hexenewCH3CH2CH2+CH2dCHdCH2, which can also occur but is minor as compared to theprevious reaction. This can explain the lower concentrationof propionaldehyde emitted from the cylcohexane fuel ascomparedto thehexene one.Octanecan givean hexyl radicalby scission and then follows the previous reactions byinternal arrangements (27).

Propionaldehyde presents a maximum concentration atstoichiometry for all fuels tested, obviously for the samereasons as methanol. No model correlating its exhaust

exhaust acetaldehyde ) a fuel ethanol +b fuel n-hexane + c fuel 1-hexene +

d fuel n-octane + e fuel 2-propanol + e

FIGURE3. Exhaustconcentrationsofacetaldehydeversusethanolandacetoneversus2-propanol. All experimentalpointsused(withdetectable concentration of ethanol and 2-propanol).




concentrationwith fuelcomposition is found, indicating that

this pollutant has multiple sources, more than these pre-sented here.Acroleine. Exhaust acroleine is principally producedfrom

fuel 1-hexene (Figure 4); octane probably contributes to itsformation. The addition of oxygenated compounds in thefuel R does not influence the emissions of this pollutant (avery slight decrease can be observed for the four highoxygenated content fuels), indicating that oxygenated com-pounds must slightly enhance the oxidation of acroleineprecursors. The addition of toluene to isooctane does notinfluence the quantity of acroleine formed, and the com-mercial fuel produces about the same quantity as fuel R. Asalmost all fuels give about the same exhaust concentration,this pollutant must have multiple sources.

The formation mechanism of acroleine must be thefollowing. 1-Hexene first loses an H and then is broken intwoparts(26), which are further oxidizedto acroleine. Octanecan form, after a scission, a hexyl radical that can continuetheaforesaid reactions. However,every other compoundthatgivesaC3 radicalcan participate in theformationof acroleine.

The exhaust concentration of acroleine increases at leanconditions. As in the case of propionaldehyde, no modelcorrelating the exhaust concentration with the fuel composi-tion is found.

Acetone. Exhaust acetone is principally produced fromfuel 2-propanol, isooctane, hexane, octane, and MTBE (4,11, 22; Figure 5). The addition of methanol andethanol doesnot decrease the exhaust acetone, indicating that theyenhance the oxidation of acetone precursors. Aromatics do

not participate to its formation, as they decrease its exhaustconcentration. Commercial fuel produces about the sameconcentration of acetone than fuel R.

The formation mechanism of acetone is the following.2-Propanol gives acetone after an extraction of an R hydrogen(22). Hexane, octane, and isooctane must first produce asecondary C3 radical which is further oxidized to CH3-COHCH3. The secondary C3 radical can come after thereactions. Hexane loses a hydrogen and is then broken intwo parts: CH3CH2CH2 and CH2dCHCH3 (26). The first onecan then give CH3CHCH3, which is further oxidized toacetone. Octanecan give, by scission, a hexylradical whichfollows the previous reactions (26). Isooctane can give,directly, a secondary C3 radicalafter thereaction CH3C(CH3)2-CH2CH(CH3)2 w CH3C(CH3)2CH2+CH3CHCH3 (28). MTBEmust give a tertio-butyl radical, which loses a methyl to giveCH3CHCH3.

influences the exhaust concentration of this pollutantin a complex way. Acetone increases with in the case of2-propanol-blended fuels (due to oxidation of 2-propanol toacetone) and presents a maximum at stoichiometry in thecase of the fuels hexane, octane, isooctane, MTBE5, andMTBE20, which arethe fuels containing thecompounds thatenhance its formation. The reasons must be that, in leanconditions, the precursors of acetone are rapidly oxidizedand, in rich conditions, they are preferably transformed toCO. An increase of exhaust concentration of this pollutantat lean conditions is observed in the case of all other fuels.

The model exhaust acetone ) a fuel 2-propanol + bfuel hexane + c fuel octane + d fuel isooctane + e fuel

FIGURE4. (Bottomgraph)Fueleffectonpropionaldehydeemissionsatstoichiometry: (mean)meanexhaustconcentration;(min,max)minimumandmaximumconcentrationfora95%confidence level.(Middle)Fueleffectonacroleineemissionsatstoichiometry.(Uppercurves) effect for some representative fuels.

FIGURE 5. (Bottomgraph) Fuel effect on acetone emissions atstoichiometry: (mean) mean exhaust concentration; (min, max)minimumandmaximumconcentrationfora95%confidence level.(Middle)Fuel effectoncrotonaldehydeemissionsatstoichiometry.(Upper curves) effect for some representativefuels.




MTBE + f is valid but only at stoichiometry and richconditions. The r2 of the line ,predicted values. ) a1 ,experimental values. + b1 is 0.83, with a1 ) 1.02 and b1) -0.028, indicating a quite good accordance betweenpredicted and experimental values. According to this model,at stoichiometry and for equal contents of fuelscomponents,the 15% of exhaust acetone comes from fuel 2-propanol, the3.7%fromfuelMTBE,andlessthan2%eachfromfuelhexane,isooctane, and octane (Table 3). The rest (77%) comes fromallthe other fuel components. Thepercentage of 2-propanolincreases with , while the contribution of all other fuel

compounds decreases (Table 3). No such model is valid atlean conditions.

As in the case of acetaldehyde, the addition of 2-propanolin the fuel R increases exhaust acetone linearly: exhaustacetone ) a1 fuel 2-propanol + b1 (with a1 ) 0.158 and b1) 0.71 at ) 1.0). The exhaust concentrations of these twocompounds are also linked (Figure 3). The line exhaustconcentration of acetone ) 2.38 exhaust concentration of2-propanol + 0.85 indicates that the majority of fuel2-propanol is oxidized to acetone and that only the 1/2.7 isemitted as alcohol.

Crotonaldehyde. Crotonaldehyde is principally producedfrom fuel 1-hexene and cyclohexane (Figure 5). Hexane andoctane contribute slightly to its formation. No significantdifferences are observed between iC8 and iC8T fuels,

indicating that isooctaneand toluene participate equivalentlyin its formation. The addition of oxygenated compounds inthe fuel R does not influence its emission, indicating thatthey enhance the oxidation of its precursors. Commercialfuel produces about the same quantity as fuel R.

Crotonaldehyde needs a straight chain of four carbons;hexane, 1-hexene, and octane can give a primary C4 radicalby scissions (26): hexane w CH3CH(CH2)3CH3 w CH2dCH2 + CH2(CH2)2CH3, 1-hexenewCH2dC(CH2)3CH3 wCHtCH + CH2(CH2)2CH3, octane wCH3CH(CH2)5CH3 wCH2dCH2 + CH2(CH2)4CH3 w CH2dCH2 + CH2(CH2)2CH3. Theprimary C4 radical must then give a CH3CHdCHCH2, whichis oxidized to crotonaldehyde. In the case of cyclohexane, aring-opening is first necessary. As cyclohexane enhances theformation of 1-hexene (14), the mechanism passes by the

formation of a hexyl radical and then a C4 straight chain.The exhaust concentration of crotonaldehyde increases

at lean conditions. As many fuel compounds can give C4radicals, no model correlating the fuel composition with theexhaust concentration of this pollutant is found.

Methacroleine. Exhaust methacroleine is clearly producedfrom fuel isooctane (Figure 6) by the formation of isobuteneas an intermediateproduct (4, 10). The comparison betweeniC8 and iC8T fuels show that the latter produces about the80% of methacroleine of the first one. Even if MTBE issuspected to enhance the formation of methacroleine(because both can produce a (CH3)2CdCH2 radical), ourresults show no clear tendencies between fuel MTBE andexhaust methacroleine. The addition of oxygenated com-poundsdoes notinfluencethe exhaust concentration of this

pollutant, indicating that they facilitate the oxidation ofisooctane or the intermediate radicals. Commercial fuelproducesa lower concentration of thispollutantas comparedto the fuel R, due to lower content of isooctane.

The exhaust concentration of methacroleine increases atlean conditions. The model exhaust methacroleine ) afuel isooctane is valid.The r2of the line ,predicted values.) a1 ,experimental values. + b1 is 0.94, with a1 ) 1.047and b1 ) -0.051, indicating good accordance betweenpredicted and experimental values. The addition of MTBEto this model gives a poorer correlation: the r2 is only 0.69,indicating that MTBE does not participate in the formationof this pollutant.

n-Butyraldehyde and MEK. n-Butyraldehyde and MEKcannot be separated under the analytical conditions used.

Ourresults showthat hexane, octane, andisooctane enhancetheir formation, obviouslyby theintermediate of C4 radicals.The sum of these two pollutants shows a maximum value atstoichiometry.

Benzaldehyde. Exhaust benzaldehyde is produced fromfuel aromatic hydrocarbons, as already presented (4, 14).The major source is fuel toluene, followed by ETB(Figure 6).A comparison between iC8 and iC8T fuels shows thatisooctaneproduces small quantitiesof this pollutant but thatthe majority comes from toluene. Literature presents thatbenzaldehyde exhaust emission decreases with the additionof oxygenates (21), but our results do not show this trend.The low oxygenates content fuels enhance slightly theformation of benzaldehyde, probably because of the en-hancement of aromatics oxidation. This pollutant shows a

maximum at stoichiometry for all fuels used.The model exhaust benzaldehyde ) a fuel benzene +

b fuel toluene + c fuel ETB + d fuel o-xylene is valid.The r2of the line ,predicted values. ) a1,experimentalvalues. + b1 is 0.97, with a1 ) 0.996 and b1 ) -0.007,indicating a very good accordance between predicted andexperimental values. According to this model, at stoichi-ometry and for equal fuel compound contents, the 42.6% oftheexhaust benzaldehyde comes fromfuel toluene,the 33.4%from ETB, 18.5% from o-xylene, and the rest (6%) from thefuel benzene (Table 3). This indicates that, at stoichiometry,the oxidation order is toluene > ETB > o-xylene > benzene.These percentages vary significantly with ; the first three

FIGURE6. (Bottomgraph)Fuel effectonmethacroleineemissionsatstoichiometry: (mean)meanexhaustconcentration;(min, max)minimumandmaximumconcentrationfora95%confidence level.(Middle) Fuel effectonbenzaldehyde emissions atstoichiometry.(Upper curves) effect for some representativefuels.




compounds present a maximum at stoichiometry (Table 3),while that of benzene is at a minimum at the same point.Thereasonis that,as thefirstthreearomaticsoxidizedeasierthan benzene, they give final reaction products instead ofbenzaldehyde at lean conditions. At rich conditions, due tolack of oxygen, the first three aromatics give benzene ratherthan benzaldehyde.

Influenceof Exhaust Temperature,H/C Fuel Ratio,andFuelProperties.Theexhaustconcentrationof fouraldehydesis linkedwith exhausttemperature.The lower curves of Figure7 show that, at stoichiometry, formaldehyde, acetaldehyde,and propionaldehyde decrease with exhaust temperature,

while benzaldehyde increases. No such correlation exists forthe other aldehydes. The trend of benzaldehyde is easilyexplained by the increased exhaust temperature of fuelscontaining high aromatic content. The opposite happens inthe case of the three light aldehydes. As aromatics do notparticipatein their formation,these pollutants are producedat lower exhaust temperatures. A second possible reason isthat, in higher temperatures, light aldehydes are furtheroxidized to CO, while benzaldehyde is more stable. A thirdprobablereason is thatthe threelight aldehydes are probablyformed duringthe coolingphase,where temperatureis lower.Exhaust formic acid is also linked with exhaust temperature(its concentration decreases at high temperatures; 13, 29),but no correlation is found between the exhaust concentra-tion of formic acid and those of the three aldehydes. Asexhaust methanol presents a maximum value at stoichiom-

etry, it shows a linear correlation with exhaust temperaturefor every fuel used, but no such correlation exists if all fuelsare used.

The four previous aldehydes are also linkedwith the H/Cratio of the fuel used. Upper curves of Figure 7 present that,at stoichiometry, the first three aldehydes increase with this

ratio while benzaldehyde decreases. The other aldehydes donot present such a correlation. The reasonis the same as forthe exhaust temperature influence: benzaldehyde is pro-duced from fuel aromatics which have low H/C ratio; theopposite is valid in the case of the other three aldehydes.

No correlation between the exhaust concentration of thepollutants detected and fuel physical properties (octanenumber, distillation curve, etc.) are found (using all theexperimental points or even only at lean or richconditions).

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Received for review October 17, 2001. Revised manuscriptreceived March 4, 2002. Accepted March 6, 2002.

ES010265T

FIGURE7. Correlationbetween exhaustconcentration offormal-dehyde, acetaldehyde, propionaldehyde, and benzaldehyde, and

exhaust temperature (bottomcurves) and fuel H/C ratio (uppercurves). All fuels used atstoichiometric conditions.


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