Investigation of fuel injection pattern on soot formation and oxidation processes in a light-duty diesel engine using integrated CFD-reduced chemistry

In this reported work, simulation studies of in-cylinder diesel combustion and soot formation processes under different configuration of the split-main injection scheme were presented. Numerical computation was performed by linking a plug-in chemistry solver CHEMKIN-CFD into ANSYS FLUENT package, in order to integrate chemical kinetic mechanism and Computational Fluid Dynamics (CFD) computations.

For improved computational runtime, an in-house reduced chemical mechanism was coupled with the CFD code. The reduced mechanism comprises 112 reactions with 46 species essential to diesel ignition/combustion and the formation of soot precursors and nitrogen monoxide (NO). Numerical models were first validated against experimental combustion characteristics as well as concentrations of engine-out soot and NO of a single-cylinder, light-duty diesel engine when using a split-main injection scheme.

Parameters of this injection scheme were then investigated, which included start of injection (SOI) timings, fuel mass ratios of the injection pulses and the dwell period in between injections. The key interest here was to elucidate how these affect exhaust NO and soot levels, and in particular the in-cylinder soot formation and oxidation events.

Fuel mass injected, duration of the injection and in-cylinder temperature were found to shape the soot formation and oxidation processes, which eventually produced the observed variations in the soot density at exhaust valve opening time. Based on all the simulation data from this reported work, power law was found to accurately describe the correlations between soot surface growth rate and gas mass fraction with a temperature range of 1400–2800K and an equivalence ratio values of greater than unity.

The values of both constants of the power law were highly sensitive to the in-cylinder mean gas temperatures. The findings here are helpful in providing a better understanding of in-cylinder diesel soot formation and oxidation processes in this combustion system.