Fuel Composition and Molecular Structure Effects on Soot Formation in Direct-Injection Flames Under Diesel Engine Conditions 2005-01-0381

Numerous investigations have been conducted to determine the effect of fuel composition and molecular structure on particulate emissions using exhaust gas analysis, but relatively few measurements have been obtained in-cylinder or under conditions where fuel effects can be isolated from other variables. Recent work has shown that the amount of air entrained upstream of the lift-off length is critical to soot formation and therefore must be controlled when making relative comparisons of soot formed from various fuels. In this work, dimethoxymethane was used as the base fuel to produce a non-sooting flame with relatively constant lift-off length in a constant volume combustion vessel at 1000 K, and a density of 16.6 kg/m³. A second fuel was then mixed into the dimethoxymethane (DMM) to determine a point at which soot formation begins. Line-of-sight extinction measurements of soot produced in binary fuel mixture flames was used as the primary diagnostic tool to determine if a correlation existed between soot and fuel properties such as the number of C-C bonds, C/H ratio or C/O ratio. Tests to date show that fuels with different molecular structures have the same incipient soot limit at a C/H ratio near 0.4, while further increases in C/H ratio produce a linear increase in soot concentration, but with a different slope for each fuel. Soot was first formed with the addition of 10 vol% toluene to DMM, while it took 40 vol% undecane and 50 vol% n-heptane. The incipient soot oxygen-to-carbon ratio at the assumed 12 mm lift-off length was 0.6 for toluene, 0.37 for undecane, and 0.3 for n-heptane. These data indicate that the aromatic toluene has a greater tendency to produce soot than the alkanes.