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The 4th Workshop of the Engine Combustion Network (ECN) was held September 5-6, 2015 in Kyoto, Japan. This manuscript presents a summary of the progress in experiments and modeling among ECN contributors leading to a better understanding of soot formation under the ECN “Spray A” configuration and some parametric variants. Relevant published and unpublished work from prior ECN workshops is reviewed. Experiments measuring soot particle size and morphology, soot volume fraction (fv), and transient soot mass have been conducted at various international institutions providing target data for improvements to computational models. Multiple modeling contributions using both the Reynolds Averaged Navier-Stokes (RANS) Equations approach and the Large-Eddy Simulation (LES) approach have been submitted. Among these, various chemical mechanisms, soot models, and turbulence-chemistry interaction (TCI) methodologies have been considered

Information about the polydispersity of soot can in principle be gained from time-resolved laser-induced incandescence (LII) using pre-assumed particle-size distributions. This paper introduces an alternative method, called two-exponential reverse fitting (TERF) that is based on combined mono-exponential fits to the LII signal decay at various delay times. The method approximates the particle-size distribution as a combination of one large and one small monodisperse equivalent mean particle size and does not require a distribution assumption. It also provides a ratio of the contribution of both size classes. The systematic error caused by describing LII signals by mono-exponential decays is calculated as less than 2% for LII signals simulated for monodisperse aggregated soot with heat-up temperatures for which evaporation is negligible. The effects of particle size, heat-up temperature, aggregate size, and pressure on this error are evaluated. The method is tested on simulated LII signals for lognormal and bimodal size distributions and applied to LII data acquired in a laminar non-premixed ethylene/air flame at various heights above burner. The results are compared to transmission electron microscopy (TEM) measurements of thermophoretically-sampled soot. The particle size of the large particle-size class evaluated with the method showed good consistency with TEM results, however the size of the small particle-size class and the relative contribution could not be compared due to missing information in the TEM results for small particles. These limitations of TEM measurements are discussed and the effect of the exposure time of the sampling grid is evaluated.