An investigation of diesel soot formation processes using micro-orifices

Abstract

Soot formation processes of diesel fuel jets were investigated in a constant-volume combustion vessel under heavy-duty, direct-injection (DI) diesel engine conditions using orifice diameters as small as 50 μm. Soot was measured with line-of-sight laser extinction and planar laser-induced incandescence techniques, and flame liftoff lengths were determined with time-averaged OH chemiluminescence imaging. Results show that as fuel-air mixing upstream of the liftoff length increases, the amount of soot measured within a fuel jet decreases. When the cross-sectional average equivalence ratio at the liftoff length decreases to a value less than approximately 2, soot is no longer formed within the fuel jet. The soot measurements provide direct proof of the link between soot formation and mixing of fuel and air upstream of the liftoff length previously observed using total soot luminosity measurements. The non-sooting conditions were achieved with the 50 μm micro-orifice at an ambient gas temperature and density of 1000 K and 14,8 kg/m3 and ambient oxygen concentrations between 21% and 10%. The temperature and density are typical of DI diesel in-cylinder conditions. The lack of soot for the lower oxygen concentration conditions, which have substantially lower flame temperatures, suggests that NOx and soot can potentially be simultaneously reduced with small orifices and exhaust-gas recirculation.