2003 environ. sci. technol. zervas e. emissions of regulated pollutants from a spark ignition...

7/30/2019 2003 Environ. Sci. Technol. Zervas E. Emissions of Regulated Pollutants From a Spark Ignition Engine. Influence of

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Emiss ions of Regulated Pollutantsfrom a Spark Ignition Engine.Influence of Fuel and Air/FuelEquivalence 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 is used to determine the influenceof fuel composition and air/fuel equivalence ratio onthe exhaust emissions of regulated pollutants. Two specificfuelmatricesareused: thefirst containseighthydrocarbonsand the second contains four oxygenated compounds.

A specific experimental design is used for these tests. Fuelaromatics increase the exhaust CO, HC, and NOx atstoichiometry, lean and rich conditions. Lambda is moreimportant than fuel composition in the case of CO and HC.At stoichiometry, the addition of oxygenated compoundscan decrease exhaust CO, HC, and NOx up to 30%, 50%,-and 60%, respectively. Under these conditions, theaddition of 5% of 2-propanol is the most effective for thereductionofCO,theadditionof20%ofethanolforthereductionof HC, and this of 5% of methyl tributyl ester (MTBE) fortheNOx.Theadditionofoxygenatedcompounds candecreaseCO by 30% at lean conditions, while no decrease isobserved at rich ones; HC and NOx can decrease up to30% and 80%, respectively, under lean conditions and 50%

under rich ones. At all lambda tested, exhaust NOxincreases with the addition of 20% of 2-propanol.

In t roduct ionFuel composition is one of the major parameters for the gasconcentrations of exhaust pollutantsof sparkignitionengines.The correlations between fuel composition and exhaustconcentration of the threeregulatedpollutants, CO, HC, andNOx, are presented in many articles (1-8and many others).Gasoline is mainly composed by hydrocarbons, but all itscomponents do not have the same participations to theformation of exhaust pollutants (2, 4, 5). The addition ofoxygenated compounds into gasoline is also proposed to

decreaseexhaust emissions of regulated pollutants.The mainoxygenatedcompoundsstudied are methanol (6, 9), ethanol(6, 9, 10),andMTBE (2, 6-8, 10-12, andmanyothers). Manyauthors present a decrease of exhaust emissions by theaddition of oxygenated compounds, but others present nochange (for example the addition of MTBE does not changethe emissions of CO (7, 13), HC (13), or NOx (11-13)); anincrease is even noticedin the case of NOxafter the addition

of ethanol (6) or MTBE (2, 6). These differences can beexplained by the different methods used for the precisedetermination of , especially in the older works (14-16).

The addition of oxygenated compounds, as ethanol,produced from renewable biological sources can also de-crease the emission of carbon dioxide, which is consideredtoday as one of the factors for the global climate changes.

Even if theaddition of oxygenated compoundsdecreasesgenerally the CO and HC emissions, it is not certain that theair quality improves. In one study, an increasing air quality,for CO, HC, and NOx, is observed after the introduction orincrease of oxygenate content (17), in another one, atmo-spheric NOxdo not change (18). The addition of oxygenatedcompounds increases the exhaust emission of other non-regulatedpollutants. For example, the addition of methanolincreases exhaust formaldehyde and those of ethanol in-creases exhaust acetaldehyde emissions (5, 10, 19), whichare more reactive in the atmosphere than their parentalcohols (20). The addition of oxygenated compounds alsoincreases the exhaust emission of organic acids (21).

Spark ignition engines operate under stoichiometry andmost of the authors studied these conditions. But modernengines also operate under lean or rich conditions (leanburnengines, cold start or very high loads), and the influence offuel composition under these conditions has not been wellstudied. The addition of oxygenated compounds probablydoes nothave thesame effects under lean or rich conditions.Onlyone author studies the influence of fuel/air equivalenceratio (22).

In ourprevious articles we presentedthe influenceof fuelcomposition and air/fuel equivalence ratio () on theemissions of organic acids (21), alcohols, and carbonylcompounds (19) of a SI engine. It was found that these twoparameters have a significant influence on the emissions oftheseunregulated compounds.Continuing this research, wepresent here the influence of fuel composition and on theemissions of regulated pollutants (CO, HC, and NOx). Twospecificfuel matrixes containing eight hydrocarbons andfour

oxygenated compounds are used to study the influence offuel composition and air/fuel equivalence ratio on theemissions of CO, HC, and NOx. A comparison between theoxygenated fuels is also performed.

Experimental SectionA CooperativeFuel Research Committee(CFR) spark ignitionengine was used for these tests. This engine is a smallmonocylinder engine (displacement ) 6.11 10-4 m3, bore )0.08255 m, stroke ) 0.1143 m) used for the octane numberdetermination. The usedvaries from0.83 to 1.25(calculatedfrom the exhaust gas analysis used five gases: CO2, CO, HC,NOx, and O2), while all other engine parameters were keptconstant (speed) 15 Hz, compression ratio) 6:1, indicatedmean effective pressure ) 4.5 105 Pa). Modern engines

emit lower pollutant concentrations than the CFR engine,butthis one allows the determination of themost importantcorrelationsbetweenfuel composition and exhaustemissionsof regulated pollutants. As the subject of this work was tofind out the aforementioned correlations, no catalyticconverter was used. Carbon monoxide was analyzed bynondispersed infrared, nitrogen oxides by chemilumines-cence andtotalunburned hydrocarbons bya flame ionizationdetector.

Two fuel matrixes were adopted in this study (Table 1).The first one (synthetic fuels matrix) contains eight hydro-carbons: n-hexane, 1-hexene, cyclohexane, n-octane, isooc-tane (2,2,4-trimethylpentane), toluene, o-xylene, and eth-

* Corresponding author phone: 331-69 27 84 77; fax: 331-69 2782 92; e-mail: [email protected]. Present address:Renault, CTL L26 0 60, 1, Allee Cornuel, F-91510 Lardy, France.

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

Environ. Sci. Technol. 2003, 37, 3232-3238

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ylbenzene (ETB), while the second one (oxygenated fuelsmatrix) also contains four oxygenated compounds: metha-nol,ethanol,2-propanol, andMTBE.An experimentaldesign,specially adapted for mixtures, is used to determine eachcomponent quantity in the blend of synthetic fuels (23). To

avoid a high dispersion in physical properties of the fuelsused, an alkylate, containing basically isooctane butalso 1.5%of benzene, was used as the base fuel for these blends. Thereference fuel R contains an equal content of each of theeight compounds, while the other fuels contain 42% of amajor component. The oxygenated matrix was obtained bythe addition of 5% or 20% of one of the four oxygenatedcompounds to the fuel R. Two simple fuels were also used:iC8, which is pure isooctane and iC8T, which is a mixture of80% of isooctane and 20% of toluene. These fuels allow thestudy of the addition of an aromaticcomponentto an alkylatebasis. The name of each fuel was chosen to recall its majorcomponent. The chemical composition and physical prop-erties of these fuels are quite different than the commercialones, but these matrixes allow the study of the influence ofthe chemical composition of the fuels on the emission of

regulated pollutants. More details about these fuels arepresented elsewhere (24).

All tests were doubled, and average values were used. TheCO, HC, and NOx relative standard deviation is 3.5, 8.5, and12.5%, respectively, estimated from five points of thereference fuel R.

Results and DiscussionEmissions of Synthetic Fuels at Stoichiometry, Lean andRich Conditions. CO. At stoichiometry, aromatics (1, 3, 4)andoctane enhance exhaust CO,while isooctane,1-hexene,n-hexane, and cyclohexane decrease it (Figure 1). Theaddition of 20% of toluene to pure isooctane increases theconcentration of exhaustCO. Other authors present a smallerdifference by the increase of aromatic content on the COemissions on the European driving cycle (13) or even anincrease of exhaust CO by the reduction of fuel aromatics(2). This latter author reports a small decrease of CO by thedecrease of fuel olefins from 20 to 5%.

CO emissions areenhancedat rich conditions dueto lackof oxygen and decreased at lean ones due to oxygen excess.All fuels emit from 0.06 to 0.1% of CO at lean conditions,compared to 0.3-0.5% at ) 1.0 (Figure 1). The differencesbetween ) 1.25 and ) 1.11 are negligible. Under theseconditions, hexane, isooctane, and toluene enhance theemission of CO, while, 1-hexene, cyclohexane, and octanedecrease it. At rich conditions, all fuels emit 2.8-3.1% and5.7-6.8% of CO at ) 0.91 and ) 0.83, respectively.

Aromatics still produce the higher concentrations, but thesedifferences are lower than at) 1.0. The addition of toluene

TABLE 1. Chemical Analysis and Octane Number of the Fuels Used (% Vol)

hexane hexene cyclohexane octane isooctane tolueneo-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 4 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 3.60

alkylate 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 93.1H/C 2.19 2.06 2.06 2.16 2.16 1.66 1.71 1.72 1.95 2.04 2.02 2.01 2.01 2.23 2.14 2.10 2.06O/C 0.028 0.018 0.013 0.008 0.12 0.074 0.053 0.033

FIGURE 1. EmissionofCOforthe fuels of the, synthetic matrix.,atstoichiometry(middlebars),lean(lowerbars),andrich(upperbar)conditions.Mean: meanexhaustconcentration,minandmax:min and max exhaust concentration for a 95% confidence level.

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into pure isooctane under lean conditions does not changeCO emissions, while it increases them under rich ones. Thedifferences between CO emissions of the fuels used are not

very important at rich conditions. These effects are sum-marized in Table 2.

The value of is more important than fuel compositionfor the emission of this pollutant. The differences betweenCO emitted from all synthetic fuels are less than 40%, 25%,and 10% under lean, stoichiometric, and rich conditions,respectively. Concerningthe influence, exhaust COis 6-10and 12-20 times more important at rich conditions than atstoichiometry (for ) 0.91 or ) 0.83, respectively), whileit is 3-6.8 times lower than at ) 1.0 at lean ones.

HC. At stoichiometry, the emission of exhaust hydrocar-bons is enhanced by fuel aromatics (1-4) except ETB anddecreased by all other fuel components (Figure 2). Theaddition of toluene to pure isooctane enhances slightly theexhaust concentration of HC. Another author presents a

smaller influence on exhaust HC by the increase of aromaticfuel content on the European driving cycle (13), and nochange of HC by the decrease of fuel olefins from 20 to 5%(2).

As in the case of CO emissions, HC emissions are enhancedunder rich conditions due to lack of oxygen and decreasedunder lean ones. Under these latter conditions, the HCexhaust concentrations vary only from 155 to 326 ppmv(Figure 2), compared to 606-1131 ppmv at ) 1.0. Tolueneand o-xylene still produce the higher concentrations, while1-hexene and cyclohexane produce the lowest ones. Underrich conditions, the HC concentrations vary from 1034 to2131 ppmv. Toluene and o-xylene still enhance theseemissions,while 1-hexene andcyclohexaneproduce the less.The additionof toluene intopure isooctaneincreases slightlytheHC emissions compared to pure isooctane,for both leanand rich conditions. These effects are summarized in Table2.

As for CO, lambda is generally more important than fuelcomposition for the HC emissions. The differences betweenHC emitted from all synthetic fuels are less than 30% underall tested. Concerning the influence of , exhaust HC is1.3-1.7and 1.9-2.2timesmoreimportantat rich conditionsthanat stoichiometry (for)0.91and )0.83, respectively),while they are 2.9-3.8 and 3.8-5 times lower than at ) 1.0at lean ones ( ) 1.11 and 1.25, respectively). All thesevariations are less important than those of exhaust CO.

NOx. It has been reported that exhaust NOx is enhancedby fuel aromatics at ) 1.0 (1-4). The exhaust temperature

TABLE 2. Inf luence of Each Fuel Component on the CO, HC, and NO x Exhaust Emissionsa

CO HC NOx

1.25 1.11 1 0.91 0.83 1.25 1.11 1 0.91 0.83 1.25 1.11 1 0.91 0.83

hexane - - D - D D - - - D E - - - -1-hexene D D D - D D D D D D D D D D Dcyclohexane D D D - - D D D D D D D D D Dn-octane D D - D - D D D D D D D D D Disooctane - - D D D - E - E - E D D - -toluene - - E - - E E E E E E E - E Eo-xylene D D E - E - E E E E E E - E EETB D D E - - D D D - D - E - E -M5 D D - - - D D D D D D D D D DE5 D D D - - D D D D D D D D D DIP5 D D D - - D - D D D D D D D DMTBE5 D D - - - D D D D D D D D D DM20 D D - - - D D D D D D D D - DE20 D D D - D D D D D D D D D D DIP20 D D D - - - - D D D E E E E EMTBE20 D D D - D D D D D D D D D D E

a E ) enhance more than 5% relatively to fuel R, D ) decrease more than 5% relatively to fuel R, - ) no influence (enhance or decrease lessthan 5%).

FIGURE 2. EmissionofHC for the fuels of the, synthetic matrix.,atstoichiometry(middlebars),lean(lowerbars),andrich(upperbar)conditions.Mean: meanexhaustconcentration,minandmax:min and max exhaust concentration for a 95% confidence level.

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of aromatic fuels is higher than that of the others fuels, soNOx emission is higher (25). Our results show that hexane,toluene, o-xylene, and ethylbenzene enhance very slightlyexhaust NOx(Figure 3 (13)), while the lowest concentrationscome from 1-hexene.The addition of 20%of toluene to pureisooctane also increases the exhaust concentration of NOx.Another author presents a smaller influence on exhaust NOxby the increase of aromatic fuel content on the Europeandriving cycle (13) or presents no change by the decrease offuel olefins from 20 to 5% (2).

NOx emissions present a maximum around ) 1.1,because at this point, the combination between flametemperature and oxygen concentration is more favorable,even if flame temperature presents a maximum at stoichi-ometry. The highest NOx emissions are emitted at ) 1.11(Figure 3, from 529 to 1560 ppmv, 1.1-1.5 times more thanat) 1.0). Toluene, o-xylene, ETB, andhexane still enhancethese emissions,while 1-hexeneemits thelowest ones.Thesedifferences are more important than at ) 1.0. The sameresults are observed at ) 1.25, where NOx emissions varyfrom 264 to 836 ppmv. Atrich conditions, NOxemissions aremuch lower than at stoichiometry: from 183 to 507 ppmvat ) 0.91 and from 67 to 157 ppmv and ) 0.83. The

addition of toluene into pure isooctane increases NOxemissions compared to pure isooctane, for both lean andrich conditions. These effects are summarized in Table 2.

Both and the type of fuel are important parameters forNOx emissions. The differences between NOx emitted fromall synthetic fuels are less than 60% under all used.Concerning the influence of, NOx is 2-3 and 6-10 timeslower at rich conditions than at stoichiometry (for ) 0.91and ) 0.83, respectively). At lean conditions, they are 1.1-1.5 times more at ) 1.11 than at stoichiometry, while theyare 1-2 times less at ) 1.25.

For the three pollutants, aromatics increase exhaustemissions, while1-hexene and cyclohexane produce the lessones. Fuel composition is much less importantthan lambdafor the CO and HC emissions, especially under rich condi-tions.

Emissions from Oxygenated Fuels at Stoichiometry,Leanand RichConditions.CO. Figure 4 presents the averagechange of CO emissions due to addition of oxygenatedcompounds into fuel R. The addition of methanol decreases2.5-5% the exhaust CO at stoichiometry, with no significantdifferences between M5and M20fuels. Literature presentsalso a decrease at) 1.0 from methanol-containing fuels (6,9). Thisadditiondecreasesexhaust COalso atlean conditions.This decrease is more important than at ) 1.0 and can

FIGURE3. EmissionofNOxfor thefuelsofthe, synthetic matrix.,atstoichiometry(middlebars),lean(lowerbars),andrich(upperbar)conditions.Mean: meanexhaustconcentration,minandmax:min and max exhaust concentration for a 95% confidence level.

FIGURE4. ChangeofCOemissionsfromtheadditionofmethanol,ethanol, 2-propanol, and MTBEfor the five used. The curves of ) 1.25and ) 1.11are superposed.



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reach 30% in the case of M5fuel but less (20%) in the caseof the M20 one. The changes are the same in the case of ) 1.25 and ) 1.11. At rich conditions, exhaust CO remainspractically unchanged (differences between -4% to 2%).

Theaddition of 5 or 20%of ethanol decreases exhaust COby 20% at stoichiometry, in accordance with literature (5, 6,9, 26, 27). Atlean conditions, this decrease is more importantin the case of E5 fuel reaching 30%, while the E20 fueldecreases it by20%. At richconditions, theadditionof ethanoldoes not influence significantly exhaust CO emissions(differences between -7% and 1%). Our results are not in

accordance with those reported previously by other inves-tigators, i.e., the addition of ethanol has no effect on COemissions under lean conditions, while this decrease canreach 20% under rich ones for an E20fuel (22). No obviousinterpretation can be given to this difference; the onlyprobable one is the presence of 7.8% of water in the alcoholused in this latter study, that can affect the miscibility ofalcohol/gasoline.

In the case of P5 fuel at stoichiometry, the addition of2-propanol decreases exhaust CO by 28%, against 8% in thecaseofthe P20one.Literature presentsalso a decrease(about50% at ) 1.0 and idle conditions from a P10 fuel (6)). Atlean conditions, P5fuel decreases exhaust CO by the samepercentage,while the P20onereaches 20%. At richconditions,exhaust CO remains unchanged by the addition of this

oxygenated compound (differences form -3 to -0.3%).The addition of MTBE decreases slightly the emission of

CO at ) 1.0 (less than 8%). Literature presents a decreaseof CO emissions at stoichiometry by the addition of MTBE(1-3, 6, 11, 12, 26, 28, 29) or no change (7, 13). Anotherauthor presents that a significant decrease of exhaust COfrom the addition of MTBE occurs only under high engineloads (8). Figure4 shows that this decrease is more importantin the case of lean conditions reaching 30% for both MTBE5and MTBE20fuels. At rich conditions, exhaust CO remainspractically unchanged by the addition of MTBE (differencesfrom -6% to 1%).

Comparing these four oxygenated compounds in case ofstoichiometry, themost effective fuelis theP5with a decreaseof 28%, followed by the E5 (-20%), while the addition of

methanol or MTBE decreases exhaust CO very slightly. Atlean conditions, there is practically no difference between ) 1.25 and ) 1.11, and it must be noticed that lowoxygenate content fuels lead to a greater decrease of COemissions than the high oxygenated content ones (30%against about 20%). This decrease seems to be independentof theoxygenatedcompound.At richconditions,the additionof oxygenated compounds does not influence significantlythe CO emissions. All four oxygenated compounds emit lessCO than the,best. syntheticfuel forthe CO emissions(theisooctane one) under lean conditions (20-30%), but theyemitmore under stoichiometryand richconditions (3-30%).These effects are summarized in Table 2.

At stoichiometry and lean conditions, the decrease ofexhaust CO due to the addition of oxygenated compoundsis more important than the percentage of this compound inthefuel. Thisstatementindicates thatthe decreaseof exhaustCO comes not only because of a dilution of the fuel but alsothat the addition of oxygenated compounds enhances thecombustion of CO in the cylinder or during the postcom-bustion processes. This must also be the reason of theunchanged emissions of CO between reference fuel and theoxygenated ones at rich conditions.

HC. Figure5 presentsthe average changeof HC emissionsdue to the addition of oxygenated compounds in the fuel R.At stoichiometry, exhaust HC decreases up to 18% by theaddition of 5% of methanol and 29% using the M20 fuel.Literature presents almost similar results at stoichiometry(6). For the five used, the addition of 5% of methanol

decreases exhaust HC almost by the same percentage (18-24%). The changes from, the M20, fuel are slightly moreimportant (21-29%) but seem again independent of the .

At stoichiometry, the addition of ethanol decreasesexhaust HC by 12%and 48%, respectively,for theE5and E20fuels. Literature presents almost the same results at stoi-chiometry (5, 6, 26, 27). Figure4 shows that thedecrease dueto E5fuel is almost independent of and remains at 7-9%(except at ) 1.25 where it reaches 19%), while this ofE20presents twozones: leanconditions with a decrease of 9-28%andstoichiometry andrich conditions witha moreimportantdecrease, 46-48%. Our results are not in accordance withthose already reported. Literature presents a very slightdecrease (3-5%) from the addition of ethanol up to 20% inthe same range of (22). No obvious interpretation can begiven to this difference except this given previously in thecase of CO.

The addition of 2-propanol influences little exhaust HC:a decrease of only 6% at stoichiometry. Literature presentsalso a decrease from the use of a P10fuel (6). For all five used, this decrease is from 4 to 8% in the case of P5fuel and1 to 13% in the case of P20. As for the addition of ethanol,the decrease due to addition of 20% of 2-propanol is higherunder richthanunderlean conditions (5-13%against 1-2%).

The addition of MTBE decreases, at ) 1.0, exhaust HCby 20% and 12%, respectively, for the MTBE5and MTBE20

FIGURE5. ChangeofHCemissionsfromtheadditionofmethanol,ethanol, 2-propanol, and MTBE for the five used.



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fuels.Literaturepresentsa decrease of exhaust HC by5-20%after the addition of MTBE (1, 2, 3, 5, 6, 7, 11, 12, 26, 28, 29)or no significant difference (13). Another author shows thata significant decrease of exhaust HC from the addition ofMTBE occurs only under high engine loads (8). Figure 5presents that this decrease is quite independent of ; itremains from 18 to 25% in the case of MTBE5fuel and from7 to 18% for the MTBE20 one, for the five used. Nodifferencescanbe observed between richand leanconditions.

At stoichiometry, the most effective fuel for the decreaseof exhaust HC is the E20, which decreases them by almost

50%, following by the M20 with a decrease of 30%. At leanconditions,the differences between) 1.25 and) 1.11 aregenerally small. The best oxygenate is methanol (decrease20-29%) followed by ethanol (10-30%), MTBE (10-18%),and 2-propanol (1-9%). At rich conditions, the best fuel isthe E20 one (46-47%), followed by methanol (18-28%),MTBE (7-25%), and 2-propanol (5-13%), while the E5fueldecreases exhaust HC only by 7-9%. All four oxygenatedcompounds emit5-38%moreHCthanthe,best. syntheticfuel for the HC emissions (the hexeneone), except two fuelsunder stoichiometric and rich conditions: the M20one thatemits comparable concentrations and the E20that emit 22-29% less that the hexeneone. These effects are summarizedin Table 2.

The decrease of exhaust HC is more than 5% in the case

of lowcontent oxygenated fuels,indicating that theadditionof an oxygenated compound enhances the combustion ofHC or their postoxidation. This is not always the case for theaddition of 20% of an oxygenated compound. The additionof methanol or ethanol generally decreases exhaust HC bymore than 20%, while this of 2-propanol or MTBE decreasesthem less, indicating that only the first two oxygenatedcompounds enhance the combustion of HC.

NOx. Theaverage change ofNOxemissions dueto additionof oxygenated compounds is presented in Figure 6. Atstoichiometry, theaddition of 5 or 20%of methanoldecreasesexhaust NOxby 18% and7%, respectively.Literature presentsalso an increase at stoichiometry froma M10fuel(6).For thefive used, the additionof 5% of methanol decreases exhaustNOx by 11-27%, with the more important decrease under

lean conditions (24-

26% against 11-

17% under rich ones).The addition of 20% of methanol changes very little exhaustNOx (decrease of 2-8% for all used).

The addition of 5% or 20% of ethanol decreases exhaustNOxby22% and19%, respectively,at)1.0. At stoichiometry,literature presents an increase from the addition of ethanol(5, 6) or a decrease of 5-25% on the European cycle usingan E5one (26). For the five used, the addition of 5% or 20%of ethanol in the reference fuel decreases NOx by 15-30%.Generally, these changes are more important at lean condi-tions. Literature presents thatthe use of an E10fuel decreasesslightly exhaust NOx (3-7%), while this of an E20 oneincreases them; an exception is observed at very leanconditions, while even the E20 fuel decreases exhaust NOx(22).

The additionof 2-propanol decreasesexhaustNOxby31%in the case ofP5fuel, while it increases them by 17% in thecase of the P20 one (at ) 1.0). An increase of 5-10% isalready presented in the case of a P10 fuel (6). For the five used, fuel P5decreases NOx by 27-38%, while the P20oneincreases them by 15-34%. In the last case, the increaseobserved is more important at lean conditions.

The addition of 5% or 20% of MTBE at stoichiometrydecreases exhaust NOx by 60% and 18%, respectively.Literature presents a decrease of 7-12% using MTBE15(7,26); other authors found that the addition of MTBE does notdecrease significantly NOx emissions (1, 5, 11-13, 28, 29);others present even an increase of 5-15% by the additionof10-12%ofMTBE(2, 6). These differences can be explained

bythe different techniquesused forthe precisedeterminationof. Figure 6 presents that, for the five used, the additionof 5% of MTBE decreases exhaust NOx by 51-82%. Thisdecrease is more important at lean than at rich conditions(63-82% against 50-51%). In the case of MTBE20fuel, thedecrease is less important, from -6 (a small increase in thecase of)0.83)to 54%. There is no clearcorrelationbetweenthe NOx emissions of the oxygenated fuels and the ,best.synthetic one (the hexene). All four oxygenated compoundsemit 2-82% less NOx at ) 1.25 (except the P20one). TheMTBE5emit 20-82% less NOx at all five used. The otherfuels emit generally more NOx than the hexene one (from 1to 114%).

Generally, theaddition of 5% of oneof thefour oxygenatedcompounds tested decreases exhaust NOx more than theaddition of 20% (except for the addition of ethanol, wherethe decrease is about the same). At stoichiometry, amongthefour lowcontent oxygenated compounds used, themosteffective one for the decrease of NOx is the MTBE5 (59%),followed by the P5(31%), E5(22%), and M5(around 18%).At lean conditions, MTBE5 is still the most effective one (65-83%), followed byP5(33-35%), E5(24-29%), and M5(24-26%). This classification remains unchanged at rich condi-tions(MTBE5: 49-51%, P5: 27-37%, E5: 15-30%, M5: 11-17%). These effects are summarized in Table 2.

FIGURE6. ChangeofNOxemissionsfromtheadditionofmethanol,ethanol, 2-propanol, and MTBE for the five used.



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Theaboveresults show that theuse of an oxygenated fuel

must take into account the global decrease of the threepollutants; themost effective fuel for theCO reduction is P5,while the E20 one is for HC and MTBE5 for NOx.

Relationsbetween Exhaust Emissionsand H/Cand O/CFuel Ratio. Literature presents a decrease of exhaust CO andHC with increasing fuel H/C fuel ratio at stoichiometry ( 4,6). Figure 7 shows that CO emissions decrease very slightlywith the increasing H/C and O/C fuel ratio, while the HCemissions decrease with both these ratios. No correlation isfound between NOx emissions and fuel H/C or O/C ratio,even if literature presents that NOx emissions decrease withincreasing H/C ratio (7).

The r2 of the best fitted lines of Figure 7 decreases withincreasing for both CO and HC in the case of fuel H/C andO/C ratio, indicating that the linearity of these correlationsis better at rich conditions. The slope aof the line y) ax+

b, with y) exhaust concentration of COor HC and x) fuelH/Cor O/C ratio, is lower at rich conditions (ais negative),indicating that the decrease of exhaust CO or HC due toincrease of fuel H/C or O/C ratio is more important at richthan at lean conditions. The change of exhaust HC is muchmore important than that of CO.

Models. In two previous articles (19, 21), we presentedseveral linear models linking the exhaust concentration ofa number of pollutants (organic acids, aldehydes, andalcohols) with the fuel composition. The same models wereconstructed in the case of CO, HC, and NOx emissions, butthe results were not satisfactory. The r2 of these models arevery low (less than 0.7), indicating than the correlationsbetween the exhaust concentrations of regulated pollutantsand fuel composition are not linear. The links between

exhaust concentration and physical properties of the fuels(octane number, distillation curves, ...) were also tested, but

the obtained results were also poor. The models proposedby Jeffrey (2) were also tested but without success. No linksare found between the CO, HC, and NOx emissions and thenumber and type of oxygen-carbon or hydrogen-carbonbonds of the fuels.

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Received for review November 12, 2002. Revised manuscriptreceived March 21, 2003. Accepted May 12, 2003.

ES026321N

FIGURE7. Influence offuel H/C andO/C ratiosontheemissionofCOand HC. All fuels used for the five.


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