Determination of soot onset and background particulate levels in a spark-ignition engine

An experimental method utilizing premixing of the air and fuel well upstream of the intake port in conjunction with careful exhaust sample conditioning was developed to study the onset of sooting in spark-ignition engines. By fully premixing the air-fuel mixture upstream of the intake port, it was possible to create a fully premixed and pre-vaporized (PMPV) fuel-air charge and to eliminate liquid fuel in-cylinder. A baseline particle size distribution (PSD) was demonstrated using multiple fuels including gasoline, methanol, and E100. The baseline particulate level was shown to be insensitive to fuel composition and equivalence ratio below a critical value of the equivalence ratio or C/O ratio (Φc or C/Oc), indicating that the baseline PSD is not the result of fuel carbon, but instead is the result of other particulate sources, e.g., engine oil and engine wear particles. With the baseline particle levels established the PMPV technique was applied to determine the local enrichment needed to generate significant soot above the baseline particle size distribution. The critical equivalence ratio for onset of sooting under PMPV operation was determined to be between 1.345 and 1.349 (C/Oc = 0.461-0.462). The results, interpreted based on the literature on laboratory premixed flames, indicate that soot first forms by the walls at the end of combustion or slightly after during the expansion stroke. The results give for the first time an indication of the level of local-enrichment that would be expected to cause a significant increase in soot production in spark-ignition direct-injection engines operating on gasoline at moderate engine load and speed conditions.