Morphology Analysis of Wall-Deposited Diesel Soot Particles via Transmission Electron Microscope

Wall-deposition of soot particles occurs due to the interaction between spray flame and cylinder liner wall/piston surface, which can potentially affect soot morphology after the in-flame formation/oxidation processes and before the exit from engine cylinder. In order to investigate these effects, flame wall impingement was simulated in a constant volume combustion vessel and thermophoretic soot sampling was conducted for Transmission Electron Microscopic analysis. A TEM grid for the sampling was exposed to a single-shot diesel spray flame multiple times and the variation of soot morphology (concentration, primary particle diameter and aggregate gyration radius) among the multiple exposures was compared. Furthermore, a newly designed impingement-type sampler vertically exposed the grid to the spray flame and sampled soot particles under different boundary condition from that of conventionally used skim-type sampler. The morphology of soot particles sampled by the impingement- and skim-type samplers were compared. The results showed that the multiple exposures increased both concentration and aggregate size but did not affect the primary particle size. On the other hand, flame wall impingement decreased both aggregate and primary particle sizes. It is suggested that cycle-to-cycle flame exposures to the wall cause soot coaggregation on wall surface but do not promote soot surface growth.