C1-C5 organic acid emissions from an SI engine: influence of fuel and air/fuel equivalence ratio.

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.