Environ. Sci. Technol. 2001, 35, 2746-2751
C1-C5 Organic Acid Emissions from an SI Engine: Influence of Fuel and Air/Fuel Equivalence Ratio E . Z E R V A S , * ,†,‡ X . M O N T A G N E , † A N D J. LAHAYE§ Institut Franc¸ ais du Pe´trole, 1 et 4 avenue du Bois Pre´au, F - 92500 Rueil-Malmaison Cedex, France and Institut de Chimie des Surfaces et Interfaces, 15 rue Jean Starcky, F - 68057 Mulhouse Cedex, France
This paper presents the influence of gasoline composition on the emissions of organic acids from a spark ignition engine. Specific fuel blends, containing hydrocarbons and oxygenated compounds, and a commercial fuel are used for this study. Exhaust organic acids are measured using specific analytical methods. As lean conditions are currently in use in commercial SI engines, the influence of the air/fuel equivalence ratio (λ) is also studied. Relations between exhaust organic acids and fuel parameters and other exhaust compounds are also researched. On the basis of the obtained results, some likely formation paths of these compounds are proposed.
Experimental Section A spark ignition engine is used to study the impact of fuel composition and of the air/fuel equivalence ratio on exhaust emissions of organic acids. Fuel blends are composed from eight hydrocarbons (n-hexane, 1-hexene, cyclohexane, n-octane, 2,2,4-trimethylpentane, toluene, o-xylene, and ethylbenzene) and four oxygenated compounds (methanol, ethanol, 2-propanol, and MTBE). Exhaust formic acid is slightly enhanced from aromatics and oxygenated compounds; acetic acid is slightly enhanced from the oxygenated fuel components; propionic acid comes from fuel aromatic compounds, and butyric acid originates from fuel o-xylene. Acrylic and isovaleric acids are also detected in lower concentrations. It is unlikely that oxygenated compounds are precursors to the formation of organic acids, but they facilitate their formation because they facilitate the oxidation of other fuel components. Exhaust concentration of formic acid is also related to exhaust oxygen and exhaust temperature. Air/fuel equivalence ratio increases the exhaust concentration of formic, acetic acid (for the fuels without oxygenated compounds), and acrylic acid and decreases the concentration of isovaleric acid. The acetic (for the oxygenated fuels), propionic, and butyric acids are at a maximum at stoichiometry.
Introduction Correlations between fuel composition and exhaust emissions from SI engines have been extensively researched for the case of regulated pollutants (refs 1-4 and many others). However, exhaust gas contains a number of specific pollutants, such as aldehydes, alcohols, and organic acids, that have thus far not been thoroughly investigated. Organic acids contribute to acid rain formation (5, 6), and many articles describe their distribution and reactions in urban, rural, and marine atmospheres (6-14). Atmospheric organic acids come from many sources, natural or anthropogenic (7), and one of which is exhaust emissions (14, 15). Few articles report the emission of these compounds from internal combustion engines (15-19) or from the combustion of propane on a flat burner (20). * Corresponding author phone: 331-69-27-84-77; fax: 331-6927-00-49; e-mail:
[email protected]. † Institut Franc ¸ ais du Pe´trole. § Institut de Chimie des Surfaces et Interfaces. ‡ Present Address: Renault - 66123 - CTL L26 0 60, 1, Alle ´e Cornuel, F - 91510 Lardy, France. E-mail: efthimios.zervas@ renault.com; tel: 331-69 27 84 77; fax: 331-69 27 00 49. 2746
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Engine and Operating Conditions. A CFR spark ignition engine (a small experimental monocylinder engine with displacement: 6.11 × 10-4 m3, bore: 0.08255 m, and stroke: 0.1143 m) is used for these tests. This type of engine is used to determine the fuel’s octane number and usually runs under two speeds: 10 and 15 Hz (600 and 900 min-1). The air/fuel equivalence ratio used is from 0.83 to 1.25. For these tests, all other engine parameters are kept constant [speed ) 15 Hz, compression ratio ) 6:1, indicated mean effective pressure (IMEP) ) 4.5 × 105 Pa]. As the subject of this work is to find the correlation between fuel composition and exhaust organic acids, no catalytic converter is used for these tests. Fuels Used. Two fuel matrixes are adopted in this study: the first matrix (called ,synthetic fuels matrix.) contains eight representative hydrocarbons: n-hexane, 1-hexene, cyclohexane, n-octane, 2,2,4-trimethylpentane, toluene, oxylene, and ethylbenzene; the second matrix (called ,oxygenated fuels matrix.) also contains four oxygenated compounds: methanol, ethanol, 2-propanol, and MTBE. Small quantities of benzene (between 0.53 and 0.66%) are also present in all fuels tested. An experimental design was used to determine the quantity of each component in the blend of ,synthetic fuels.. To avoid very disperse physical properties of the fuels used, an alkylate (containing basically isooctane) was used as the base fuel for these blends. Fuel R (the reference fuel), contains an equal content of each of the eight compounds and is the central point of the experimental matrix. Each other fuel contains 42% of a major component. These matrixes allows one to compare directly each fuel with the reference fuel; if the exhaust concentration of a pollutant is greater than in the case of the reference fuel, one can deduce that the major component of this fuel enhances the formation of the pollutant in question. The ,oxygenated. matrix was obtained by the addition of 5 or 20% of one of the four oxygenated compounds to the fuel R. Two other fuels were also used. The first fuel, i8, is pure 2,2,4-trimethylpentane, and the second, i8T, is a mixture of 80% 2,2,4-trimethylpentane and 20% toluene. These fuels allow one to study the addition of an aromatic component to an alkylate basis. Finally, a commercial gasoline was also tested. The chemical composition and the research octane number (RON) of each fuel is presented in Tables 1 and 2. More details about these fuels (distillation characteristics) may be found elsewhere (17). Analysis of Exhaust Gas. Carbon monoxide and carbon dioxide were analyzed by nondispersive infrared, nitrogen oxides were analyzed by chemiluminescence, and total unburned hydrocarbons were analyzed by a flame ionization detector (FID). Exhaust individual HC, aldehydes, and 10.1021/es000237v CCC: $20.00
2001 American Chemical Society Published on Web 06/02/2001
TABLE 1. First Fuel Matrix “Synthetic Fuels” (% vol contents) S1
S2
S3
S4
S5
S6
S7
S8
R
hexane 42 2 2 2 2 2 2 2 7 1-hexene 2 42 2 2 2 2 2 2 7 cyclohexane 2 2 42 2 2 2 2 2 7 n-octane 2 2 2 42 2 2 2 2 7 isooctane 2 2 2 2 42 2 2 2 7 toluene 2 2 2 2 2 42 2 2 7 o-xylene 2 2 2 2 2 2 42 2 7 ethylbenzene 2 2 2 2 2 2 2 42 7 alkylate 44 44 44 44 44 44 44 44 44 RON 63.7 83.7 88.7 43.6 93.8 101.3 96.6 100 85.2
alcohols were also measured, but these results will be presented in a following article. Organic acids were collected by passing a sample of raw exhaust gas through two impingers in series, each containing 20 mL of deionized water. Typical collection times are approximately 20 min. The collection efficiency of this method is over 90%. The final solution was analyzed by two methods: ionic chromatography for the analysis of formic acid, and gas chromatography for the analysis of heavier acids. More details about this method can be found in the literature (20). A standard solution containing 12 organic acids [formic (methanoic), acetic (ethanoic), propionic (propanoic), acrylic (2-propenoic acid), isobutyric (isobutanoic), butyric (butanoic), isovaleric (isopentanoic), valeric (pentanoic), isocaproic (iso-hexanoic), caproic (hexanoic), heptanoic, and benzoic] were used for chromatograph calibration and for the identification of each acid. Six acids: formic, acetic, propionic, acrylic, butyric, and isovaleric, were found in the exhaust gas in detectable concentrations. Repeatability Tests. At each air/fuel equivalence ratio, five identical points of the fuel R were used to evaluate the repeatability of the engine and the analytical methods used. All other tests were repeated, and average values were used. The relative standard deviation of the concentration of the regulated pollutants was found to be less than 1%. This value is below 12% for most of the organic acids detected, but it reaches 20-30% for the propionic and butyric acid at high λ.
Results and Discussion Emission of Regulated Pollutants. The results of the regulated pollutants at stoichiometry show that (i) CO is increased by the increasing aromatic content in the fuel, and decreased by the increasing content of hexane, 1-hexene, isooctane, and oxygenates. (ii) HC is increased by the increasing content of aromatics, and decreased by the increasing content of 1-hexene, cyclohexane, and oxygenates. (iii) NOx is increased by the increasing content of hexane, toluene, and ETB, and decreased by the increasing content of 1-hexene and the addition of oxygenates. The four fuels containing low levels of oxygenates decrease NOx emissions by a larger amount than do the fuels containing high levels of oxygenates.
Our results are in accordance with these already presented in the literature (1-4), and these findings help to validate the results of organic acids. The influence of λ on the regulated emissions are also in accordance with the literature and as are already well-known, they are not presented here. Emissions of Organic Acids. Formic acid. The bottom graph of Figure 1 shows that, at stoichiometry, all fuels produce comparable concentrations of formic acid, with aromatics, oxygenated compounds, and octane slightly enhancing the formation of this compound. The comparison between i8 and i8T fuel (pure isooctane and 80% isooctane/ 20% toluene) shows that toluene enhances the formation of exhaust formic acid more than isooctane does (also confirmed from the comparison between S5 and S6 fuels, with respectively 42% isooctane and toluene). Oxygenated compounds enhance the formation of formic acid either because they produce more precursors of this acid, or because they facilitate the formation of the formic acid precursors coming from hydrocarbons. Commercial fuel produces about the same quantity of formic acid as does the R fuel. The exhaust concentration of formic acid increases with λ for all fuels tested (some representative fuels are presented in the upper curves of Figure 1), indicating that an excess of oxygen enhances the formation of this acid. A linear correlation, but with high dispersion, between the exhaust oxygen concentration and exhaust formic acid is presented in the left graph of Figure 2. The decrease of exhaust formic acid concentration at rich conditions, must be due to the preferable formation of CO. Apparently, it is difficult for a molecule with two atoms of oxygen, as formic acid, to keep both these atoms at rich conditions, and subsequently an oxidation to CO which is a more stable product occurs. No relationship between exhaust formic acid and CO concentration was found. Figure 2 also shows that the exhaust concentration of this acid is linked with exhaust temperature (as already reported in the case of a burner, a natural gas fed SI engine and a CI engine, refs 18-20). Exhaust formic acid is linear with exhaust temperature for lean conditions and stoichiometry. Moreover, when this temperature is low, exhaust formic acid concentration is high, indicating that this acid is probably not formed during the combustion process but during the cooling phase of the exhaust gas, where formic acid comes from the oxidation of other products. The higher the temperature, the faster the formic acid is decomposed to other products and its concentration decreases. The exhaust concentration of this acid is quite independent of temperature at rich conditions. It is likely that formic acid is a product of formaldehyde oxidation as this aldehyde emission is also linked with exhaust temperature (21), but no direct link is found between formaldehyde and formic acid exhaust concentration. No relationship between formic acid and fuel H/C ratio, other physical properties of the fuel (octane number, distillation curve...) or the exhaust concentration of other compounds (CO, CO2, NOx, individual HC, aldehydes, other organic acids) are found. As formic acid is a light compound, it must have multiple sources not directly linked to the initial fuel
TABLE 2. Second Fuel Matrix “Oxygenated Fuels” (% vol contents) methanol ethanol 2-propanol MTBE fuel R oxygen (% weight) RON
O1
O2
O3
O4
O5
O6
O7
O8
5 0 0 0 95 3.08 89.4
0 5 0 0 95 1.98 90.4
0 0 5 0 95 1.45 87.5
0 0 0 5 95 0.90 86.8
20 0 0 0 80 11.89 98.6
0 20 0 0 80 7.75 97.4
0 0 20 0 80 5.72 96.6
0 0 0 20 80 3.60 93.1
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FIGURE 1. Emissions of formic acid. Bottom graph: fuel effect at stoichiometry. Mean: mean exhaust concentration; min, max: minimum and maximum concentration for a confidence region of 95%. S1 ) 42% hexane, S2 ) 42% 1-hexene, S3 ) 42% cyclohexane, S4 ) 42% octane, S5 ) 42% isooctane, S6 ) 42% toluene, S7 ) 42% o-xylene, S8 ) 42% ethylbenzene, R ) reference fuel, O1 ) 5% methanol, O2 ) 5% ethanol, O3 ) 5% 2-propanol, O4 ) 5% MTBE, O5 ) 20% methanol, O6 ) 20% ethanol, O7 ) 20% 2-propanol, O8 )20% MTBE, i8 ) pure isooctane, i8T ) 80% isooctane + 20% toluene, C ) commercial fuel. Upper curves: λ effect for some representative fuels.
FIGURE 2. Exhaust concentration of formic acid versus exhaust oxygen (left graph) and exhaust temperature (right graph) for all experimental points used. composition. Exhaust oxygen and temperature seems to be more important to the formation of this acid than initial fuel composition. No detailed mechanism, based on these results, can be proposed for the formation of formic acid. However, the addition of an OH radical to an oxygenated C1 radical seems to be the more probable path to the formation of this acid. Concerning the use of a 3-way catalyst, no detailed data are currently available; from our experience, tail pipe emissions 2748
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FIGURE 3. Emissions of acetic acid. Bottom graph: fuel effect at stoichiometry. Mean: mean exhaust concentration; min, max: minimum and maximum concentration for a confidence region of 95%. S1 ) 42% hexane, S2 ) 42% 1-hexene, S3 ) 42% cyclohexane, S4 ) 42% octane, S5 ) 42% isooctane, S6 ) 42% toluene, S7 ) 42% o-xylene, S8 ) 42% ethylbenzene, R ) reference fuel, O1 ) 5% methanol, O2 ) 5% ethanol, O3 ) 5% 2-propanol, O4 ) 5% MTBE,, O5 ) 20% methanol, O6 ) 20% ethanol, O7 ) 20% 2-propanol, O8 )20% MTBE, i8 ) pure isooctane, i8T ) 80% isooctane + 20% toluene, C ) commercial fuel. Upper curves: λ effect for some representative fuels (,synthetic. on the left and ,oxygenated. on the right). of formic acid are expected to be higher than the engine out emissions due to oxidation of other products in the converter. Acetic Acid. Figure 3 shows that, at stoichiometry, all fuels produce comparative amounts of acetic acid, with fuel cyclohexane, ethanol, and 2-propanol slightly enhancing its formation. These two oxygenated compounds enhance the formation of this acid probably because they produce more C2 radicals, or they facilitate the formation of acetic acid precursors deriving from hydrocarbons. A comparison between i8 and i8T (and also S5 and S6) fuels shows that toluene enhances the formation of acetic acid over that of isooctane, but toluene is not one of the major precursors of this compound. Commercial fuel produces a little less acetic acid than fuel R. Air/fuel equivalence ratio increases the exhaust acetic acid in the case of ,synthetic. fuels (Figure 3, upper left curve). The reasons must be the same as in the case of formic acid: excess of exhaust oxygen enhances the formation of acetic acid at lean conditions and CO formation decreases its formation when rich, but the introduction of an oxygenated compound modifies the shape of these curves at lean conditions; a maximum value at stoichiometry is now observed. Two hypothesis can be proposed for this phenomenon: (i) Exhaust temperature has a greater influence than exhaust oxygen for the formation of acetic acid in the case of ,oxygenated. fuels. The introduction of oxygenated compounds in the fuel modifies the composition of exhaust gas. The precursors of acetic acid originating from ,oxygenated. fuels are different than those that come from the ,synthetic. fuels, and they probably need a higher temperature for their oxidation to acetic acid. As this higher temperature occurs at stoichiometry, the ,oxygenated. fuels present a maximum of the exhaust acetic acid at this λ.
The maximum exhaust concentration of this acid occurs at stoichiometry (Figure 4, upper curves). The hypothesis described previously for the influence of λ on the acetic acid emissions is also valid in the case of propionic acid: the formation of CO decreases its formation at rich conditions; in the case of lean conditions, the precursors of this compound require an increased temperature for their formation, or the excess of oxygen oxidizes them. No correlation between the exhaust concentration of propionic acid to that of oxygen or other compounds, exhaust temperature, or fuel properties was found (using all the experimental points, or even only at lean or rich conditions). A quantitative model can relate the exhaust concentration of propionic acid with the percentages (in volume) of the fuel components:
propionic acid ) a benzene + b toluene + c ethylbenzene + d o-xylene The parameters a, b, c, and d are determined for all λ, but this model is valid only in the case of the engine used. The total emissions of propionic acid will probably be lower in the case of more modern engines, so the normalized model: FIGURE 4. Emissions of propionic acid. Bottom graph: fuel effect at stoichiometry. Mean: mean exhaust concentration; min, max: minimum and maximum concentration for a confidence region of 95%. S1 ) 42% hexane, S2 ) 42% 1-hexene, S3 ) 42% cyclohexane, S4 ) 42% octane, S5 ) 42% isooctane, S6 ) 42% toluene, S7 ) 42% o-xylene, S8 ) 42% ethylbenzene, R ) reference fuel, O1 ) 5% methanol, O2 ) 5% ethanol, O3 ) 5% 2-propanol, O4 ) 5% MTBE,, O5 ) 20% methanol, O6 ) 20% ethanol, O7 ) 20% 2-propanol, O8 )20% MTBE, i8 ) pure isooctane, i8T ) 80% isooctane + 20% toluene, C ) commercial fuel. Upper curves: λ effect for some representative fuels. (ii) The precursors of acetic acid coming from ,oxygenated. fuels are rapidly oxidized to other products due to the excess of oxygen at lean conditions. Our results cannot, for the moment, prove which is the correct hypothesis. No relationship between the exhaust concentration of acetic acid and exhaust concentration of oxygen or other compounds, exhaust temperature, H/C fuel ratio, or other physical properties of the fuel have been found. On the basis of these results, no detailed mechanism can be proposed for the formation of acetic acid. However, the addition of an OH radical to an oxygenated C2 radical seems to be the more probable path to the formation of this acid. No detailed data are currently available for the tail pipe emissions of acetic acid, but, as formic acid, it is expected that tail pipe emissions of acetic acid are higher than the engine out emissions due to oxidation of other products in the converter. Propionic Acid. The bottom graph of Figure 4 shows that propionic acid is clearly produced from fuel aromatics. Kawamura (15) noticed that fuel aromatics enhance the formation of dicarboxylic acids, and Zervas (19) reports that in the case of a CI engine, a hydrotreated fuel, containing more monoaromatics than a non-hydrotreated one, enhances the formation of propionic acid. To a lesser extent, ethanol and 2-propanol added at 5% in the fuel (fuels O2 and O3) also enhance its formation, probably because they facilitate the oxidation of aromatics or they enhance the formation of the C3 radicals, as can be the case for 2-propanol. The comparison between i8 and i8T fuels show the participation of toluene to the formation of this acid; pure isooctane produces minimal concentrations of propionic acid in comparison to the isooctane/toluene fuel. As the aromatics content of the commercial fuel is higher than that of fuel R, the former produces more propionic acid than the latter.
propionic acid ) a/d benzene + b/d toluene + c/d ethylbenzene + o-xylene is to be taken into consideration. According to this model, at stoichiometry, 58% of the exhaust propionic acid comes from the fuel benzene, 19% from the fuel toluene, 17% from the fuel ETB, and the remaining 6% from the fuel o-xylene. More about this model for other values of λ can be found elsewhere (17). The mechanism of this acid formation must be based on the aromatic ring opening and the formation of two C3 radicals which are further oxidized to propionic acid. The aromatic ring is probably partially oxidized to a phenol or aldehyde before opening, as presented in the following reaction.
More data is necessary to confirm this mechanism. No detailed data are currently available for tail pipe emissions of propionic acid, but the initial results show that its tail pipe emissions are lower than the engine out emissions due to its oxidation in the converter. This is also applicable for butyric, acrylic, and isovaleric acid. Butyric Acid. Evidence suggests that butyric acid is produced from o-xylene (Figure 5, bottom curve). The other two isomers of xylene (p- and m-xylene) are also probably precursors of this acid, but these two compounds were not tested in our fuel blends. All other fuels tested produce lower quantities of butyric acid; the two fuels that did not contain o-xylene (i8 and i8T fuels) did not produce it at all (less than the detection limits). As commercial fuel contains more o-xylene than fuel R, it produces more butyric acid than the latter. Air/fuel equivalence ratio has the same effect on the formation of butyric acid as propionic acid: a maximum value is observed at stoichiometry. The same remarks are valid here. No correlation was found between the exhaust concentration of this acid and exhaust oxygen or other compound concentrations, or exhaust temperature (for all the experimental points used or only lean or rich conditions). A good correlation between exhaust butyric acid concentration and exhaust temperature for the five experimental points of the VOL. 35, NO. 13, 2001 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 5. Emissions of butyric acid. Bottom graph: fuel effect at stoichiometry. Mean: mean exhaust concentration; min, max: minimum and maximum concentration for a confidence region of 95%. S1 ) 42% hexane, S2 ) 42% 1-hexene, S3 ) 42% cyclohexane, S4 ) 42% octane, S5 ) 42% isooctane, S6 ) 42% toluene, S7 ) 42% o-xylene, S8 ) 42% ethylbenzene, R ) reference fuel, O1 ) 5% methanol, O2 ) 5% ethanol, O3 ) 5% 2-propanol, O4 ) 5% MTBE, O5 ) 20% methanol, O6 ) 20% ethanol, O7 ) 20% 2-propanol, O8 )20% MTBE, i8 ) pure isooctane, i8T ) 80% isooctane + 20% toluene, C ) commercial fuel. Upper curves: λ effect for some representative fuels. o-X fuel is found:
butyric acid (ppm) ) 0.176 * temperature (°C) - 134, with r2 ) 0.81 This correlation indicates that for the same fuel, therefore the same precursors, the exhaust concentration of butyric acid depends only on the exhaust temperature. This phenomenon was also observed in the case of formic acid emitted from a natural gas fed SI engine (18). However, this correlation is not valid in the case of the other fuels for the emission of butyric acid, nor for the emissions of propionic acid from the S6, S7, or S8 fuels. As in the case of propionic acid, the following model can correlate the exhaust concentration of butyric acid with the percentages (in volume) of the fuel components at stoichiometry:
butyric acid ) a o-xylene More about this model can be found elsewhere (17). The formation mechanism of butyric acid must be similar to that of propionic acid: oxidation of o-xylene to an oxygenated compound (phenol, biphenol, or cresol), opening of the aromatic ring, and further oxidation to acid. March (ref 22, and references therein) describes the formation of an acid after a ring opening: the formation of adipic acid from cyclohexanone over CrO3 as catalyst: C6C11O w HOOC(CH2)4-COOH. It is likely that exhaust butyric and propionic acids follow a similar reaction path. Acrylic and Isovaleric Acid. Two other acids are detected in the exhaust gas of some fuels in concentrations up to 2 ppmv: acrylic and isovaleric acid. The obtained results are very scattered, but acrylic acid is enhanced from 1-hexene, cyclohexane, and octane, while isovaleric acid is enhanced 2750
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FIGURE 6. Emissions of acrylic and isovaleric acid. Effect of λ. from o-xylene and ETB (Figure 6). Acrylic acid is mainly formed at lean conditions; most of the fuels used emitted this compound in undetectable concentrations at rich conditions. Isovaleric acid follows the opposite trend: it is mainly formed at rich conditions. The commercial fuel produces almost the same concentrations of these acids as fuel R. Acrylic acid must be linked with the formation of acroleine, which is also produced from 1-hexene and octane (21). Cyclohexane enhances the formation of this acid because it enhances the formation of exhaust 1-henexe (17). It is likely that isovaleric acid is formed after the opening of the ring of o-xylene or ETB and formation of an iC5 radical, but, as already observed, o-xylene and ETB do not enhance the formation of isopentane (21); this radical must already be oxygenated before its further oxidation to isovaleric acid. This remark adds support to the hypothesis that propionic and butyric acid are produced after the ring opening of an already oxygenated aromatic compound. The commercial fuel produces almost the same concentrations of these acids as fuel R. Percentage of Organic Acids in the other Exhaust Pollutants. The quantity of each acid as a percentage of the total acids emitted for the commercial fuel at stoichiometry are formic 0.8%, acetic 24.6%, propionic 44.0%, butyric 29.6%, acrylic 1.0%, and isovaleric 0%. These percentages are calculated as the number of carbon atoms of each acid divided by the total number of carbon atoms of all acids detected. These percentages do not differ greatly from those of fuel R. The major organic acid emitted is propionic acid (due to fuel aromatics) followed by butyric (due to o-xylene) and acetic acid. Concerning the other fuels used, these percentages vary from 0.5 to 3% for the formic acid, 12-90% for the acetic, 8-80% for the propionic, 0-45% for the butyric, 0-6% for the acrylic acid, and 0-3% for the isovaleric acid. Propionic, butyric, and acetic acids are still the three major acids. Concerning the comparison between the organic acids and the other exhaust pollutants (total unburned hydrocarbons and aldehydes), all fuels used emit the following at stoichiometry: total HC (measured by FID): 457-1131 ppm, aldehydes 21-42 ppm, acids 35-291 ppm. Organic acids are 4-27% of the total HC and 1.2-10 times more than aldehydes. In the case of the commercial fuel at stoichiometry, organic acids are 24% of the total HC and 6.5 times more than aldehydes. Formic acid corresponds to 0.1% of the total HC,
acetic to 3.2%, propionic to 14.8%, butyric to 5.4%, and acrylic to 0.2% (in ppm C). The air/fuel equivalence ratio influences these ratios. Total HC decreases with λ (from 2100 to 230 ppm for the commercial fuel), while the total organic acids and total aldehydes present a maximum value at stoichiometry (acids: 276 ppmv, against 150 ppmv at λ ) 0.83 and 244 ppmv at λ ) 1.25; aldehydes: 42 ppmv, against 28 ppmv at λ ) 0.83 and 41 ppmv at λ ) 1.25). These values indicate that a more important percentage of exhaust pollutants are found in oxygenated form under lean conditions (7% at λ ) 0.83, 27% at λ ) 1.0, and 55% at λ ) 1.25).
Literature Cited (1) Jeffrey, J. G.; Elliot N. G. SAE Technical Paper Series 1993, 932680. (2) Kaiser, E. W.; Siegel, W. O.; Henig, Y. I.; Anderson, R. W.; Trinker, F. H. Envir. Sci. Technol. 1991, 25, 2005-2012. (3) Neimark, A.; Kholmer, V.; Sher, E. SAE Technical Paper Series, 1994, 940311. (4) Petit, A.; Montagne, X. SAE Technical Paper Series, 1993, 932681. (5) Keene, W. C.; Galloway, J. N. Atm. Environ. 1984, 18, 24912497. (6) Lawrence, J. E.; Koutrakis, P. Envir. Sci. Technol. 1994, 28, 957964. (7) Chebbi, A.; Carlier, P. Atm. Environ. 1996, 30, 4233-4249. (8) Granby, K.; Christensen, C. S.; Lohse, C. Atm. Environ. 1997, 31, 1403-1415. (9) Grosjean, D. Atm. Environ. 1990, 10, 2699-2702.
(10) Khare, P.; Satsangi, G. S.; Kumar, N.; Maharaj Kumari, K.; Srivastava, S. S. Atm. Environ. 1997, 31, 3867-3875. (11) Khwara, H. A. Atm. Environ. 1995, 29, 127-139. (12) Kumar, N.; Kulshrestha, U. C.; Khare, P.; Saxena, A.; Kumari, K. M.; Srivastava, S. S. Atm. Environ. 1996, 30, 3545-3550. (13) Puxbaum, H.; Rosenberg, C.; Gregori, M.; Lanzerstorfer, C.; Ober, E.; Winiwarter, W. Atm. Environ. 1988, 22, 2841-2850. (14) Souza, S.; Vasconcellos, P. C.; Carvalho, R. F. Atm. Environ. 1999, 33, 2563-2574. (15) Kawamura, K.; Kaplan, I. R. Environ. Sci. Technol. 1987, 21, 105-110. (16) Kawamura, K.; Ng, L. L.; Kaplan, I. R. Environ. Sci. Techn. 1985, 19, 1082-1086. (17) Zervas, E.; Montagne, X.; Lahaye, J. J. Air Waste Manag. Assoc. 1999, 49, 1304-1314. (18) Zervas, E.; Tazerout, M. Atm. Environ. 2000, 34, 3921-3929. (19) Zervas, E.; Montagne, X.; Lahaye, J. Atm. Environ. 2001, 35, 1301-1306. (20) Zervas, E.; Montagne, X.; Lahaye, J. Atm. Environ. 1999, 33, 4953-4962. (21) Zervas, E., Ph.D. Dissertation, University of Mulhouse, France, 1996. (22) March J. Advanced Organic Chemistry, 4th ed.; New York: J. Wiley and Sons, 1992; p 1176.
Received for review October 9, 2000. Revised manuscript received March 5, 2001. Accepted March 20, 2001. ES000237V
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