The influence of gasoline formulation on specific pollutant emissions

Many recent works have dealt with the influence of fuel composition on regu lated and specific pollutant emissions from spark ignition engines. While m any qualitative correlations have been already proposed, only a few quantit ative ones are known (benzene remains an exception). This paper describes qualitative and quantitative correlations between fuel composition and specific pollutant emissions (individual hydrocarbons, ald ehydes, ketones, alcohols, and organic acids) of a spark ignition engine. T he aim of this work was to find the precursors of the main specific polluta nts. Then, for each of them, a multilinear equation has been calculated, il lustrating the correlation between its concentration in exhaust gases and i ts content in the fuel. The results of these calculations point out which i nitial compound favors the formation of a determined pollutant. As lean con ditions are probably going to be used in future commercial engines, the fue l effect has been studied for a broad range of equivalence ratios (from 0.8 to 1.2). Two fuel matrixes were designed. The first one was obtained by introducing eight pure hydrocarbons (nC(6), nC(8), isoC(8), 1-hexene, cyclohexane, tolu ene, o-xylene, and ethylbenzene) in an alkylate base. The second one was fo rmulated to test the behavior of four oxygenated compounds (methanol, ethan ol, isopropanol, and methyl tertio-butyl ether [MTBE]). Individual unburned hydrocarbons, aldehydes, alcohols, and organic acids we re analyzed according to accurate methods developed in our laboratory. Results show that benzene exhaust is mainly emitted from benzene fuel, the substituted aromatics, and, to a lesser degree, from cyclohexane; 1,3-butad iene is produced from l-hexene or cyclohexane; and isobutene is a product o f isooctane and MTBE. Exhaust toluene comes mainly from toluene fuel, but a lso from o-xylene and ethylbenzene fuels. Isopropylbenzene exhaust seems to be produced only from ethylbenzene fuel. Exhaust concentrations of acetald ehyde and acetone showed a strong correlation with, respectively, ethanol a nd isopropanol fuel contents. Methacroleine exhaust concentration was stron gly correlated with the fuel's isooctane content. Benzaldehyde was produced by the aromatic fuels, Methanol was found in the exhaust gases of all the oxygenated fuels. High quantities of hexane or isooctane in fuels seem also to favor methane formation. Ethanol and isopropanol were found in the exha ust gases only when these components are added to the fuel. Propionic acid was produced by the aromatic fuels, while butyric acid came from o-xylene. Based on these results, this paper proposes the probable intermediate paths for formation of some of these pollutants. Finally, the ozone-forming potential of the exhaust gas of each fuel was ca lculated. Toluene and the oxygenated components of the fuel were shown to d ecrease this potential.