Dilution and injection pressure effects on ignition and onset of soot at threshold sooting conditions by simultaneous PAH-PLIF and soot-PLII imaging in a heavy duty optical diesel engine 2019-01-0553
Although in-cylinder soot can be measured by various optical techniques, discerning soot formation from oxidation is more difficult. Various optical measurements have pointed toward ways to affect in-cylinder soot oxidation, but evidence of effects of operational variables on soot formation is less plentiful. The formation of soot and its precursors, including polyaromatic hydrocarbons (PAHs), are strongly dependent on temperature, so factors affecting soot formation may be more evident at low-temperature combustion conditions. Here, in-cylinder PAH are imaged by planar laser-induced fluorescence (PAH-PLIF) using three different excitation wavelengths (355, 532, and 633 nm) to probe three different size-classes of PAH from 2-3 to 10+ rings at intake oxygen concentrations as low as 7.5%, where soot and PAH formation are at a threshold. Simultaneous planar laser-induced incandescence of soot (soot-PLII by 1064 nm laser) provides complementary imaging of soot formation near inception. Both techniques are employed at three different horizontal sheet heights to provide a rough three-dimensional representation of the in-cylinder distributions of PAH and soot.
The optical diagnostics show that increasing dilution delays the inception of large PAH by over 1.5 ms as the intake oxygen decreases from 15% to 9%. At 7.5% intake O2, no PAH or soot are formed, while the 9% intake O2 condition forms PAH but virtually no detectable soot, and conditions with 10% or more intake O2 form both PAH and soot. For these threshold-sooting conditions, large PAHs typically form throughout the cross-section of the downstream jets and along the bowl-wall. Soot appears after PAH, and typically upstream, near the jet periphery. The spatial distributions of PAH and soot overlap very little under these threshold-sooting conditions, with soot typically in upstream but not downstream regions. This may indicate that temperatures are only high enough for soot formation on the jet periphery, near the diffusion flame. The lack of overlap also suggests that PAH are largely consumed when soot is formed. Additionally, increasing the fuel-injection pressure from 533 to 1200 bar increases soot formation, which is in contrast to conventional diesel combustion. High-speed natural luminosity videos provide complementary information on the spatial and temporal progression of ignition as injection pressure is varied, providing insight into the role of mixing on PAH and soot formation under threshold sooting conditions.
Zheming Li, Gregory Roberts, Mark Musculus