The Effect of Fuel-Oil Solubility on Exhaust HC Emissions

Exhaust hydrocarbon (HC) emissions from a single-cylinder, spark-ignition engine have been measured using two liquid fuels (pure isooctane and tracer fuel: a mixture of toluene, isooctane, heptane, hexane, and pentane) and two oils (20W-40 petroleum based and a triaryl phosphate ester) in which the liquid fuels have different relative solubility. Total hydrocarbon emissions from each fuel were measured in back-to-back comparisons (i.e., only fuel changed) in each oil. The difference in total HC emissions generated by the two fuels ([isooctane] - [tracer]) decreases by 210 ± 95 ppmC1 when changing from 20W-40 to the phosphate ester lubricant. This consistent trend in hydrocarbon emissions when the oil is changed supports the contention that the presence of oil layers does affect hydrocarbon emissions. Based on results from a conceptual model, the change in relative emissions from the two fuels could be due to either an increase in the amount of oil present in the combustion chamber or a somewhat larger toluene solubility when the ester oil is used.

Laboratory measurements of the oil solubility of the fuel components present in tracer fuel at room temperature show that toluene is 28 times more soluble in 20W-40 than pentane with the other fuel components showing intermediate solubility. In the phosphate ester, the solubility of the alkanes was reduced by a factor of 3-4 relative to 20W-40 while those of the aromatic molecules remained relatively unchanged. Based on these measurements, the hydrocarbon emissions due to the aromatic content of gasoline will be more influenced by oil solubility effects than will the non-aromatic fuel components when using petroleum based motor oils.