C₁−C₅ Organic Acid Emissions from an SI Engine:  Influence of Fuel and Air/Fuel Equivalence Ratio

Abstract

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.

Ab initio and kinetic modeling studies of formic acid oxidation

“…Emissions of monocarboxylic acids have been detected from IC engines [3,4] and they are known to form in biomass pyrolysis [5]. The simplest of these acids is formic acid (HC(O)OH, or here HOCHO), which has been identified as an intermediate in oxidation…”

Effects of oxygenate concentration on species mole fractions in premixed -heptane flames

“…Emissions from two-stroke [16,17] and four-stroke engines [18][19][20][21][22][23] with oxygenated fuels have been studied extensively in the literature. The effects of blending unleaded gasoline with different proportions of MTBE (10,15, and 20 vol%) on exhaust carbon monoxide, carbon dioxide, and hydrocarbon emissions from a fixed compression ratio SI engine were studied by Osman et al [18].…”

Effect of Ethanol-Gasoline Blends on Small Engine Generator Energy Efficiency and Exhaust Emission

“…Also, the addition of ethanol could produce a significant amount of acetaldehyde. In addition, Zervas et al 13,14 suggested that organic acid exhaust (e.g., formic acid, propionic acid, and butyric acid) is slightly enhanced from oxygenated compounds.…”