Soot formation in GDI/GTDI engines

Jiao, Q. Soot Formation in GDI/GTDI Engines. University of Wisconsin-Madison, 2014.

A semi-detailed soot model was successfully implemented in the KIVA3v2-ERC code, which features a discrete multi-component (DMC) fuel vaporization model. A spark ignition model and the G-equation turbulent flame propagation model were also implemented for modeling direct-injection spark-ignition (DISI) engines. Chemistry parallelization for the soot model was also successfully realized in this work. Chemistry parallelization and a newly developed chemistry solver (SpeedCHEM) further reduced the computational time and enabled the successful application of the final code (KIVA-DMC-detsoot-G-SC) to DISI engines with the consideration of multi-component surrogates for real gasoline fuels and 3-D full cylinder engine grids.

The semi-detailed soot model considered: soot inception from a four-ring aromatic (A4), soot surface growth through acetylene (C2H2) and aromatics from single-ring to four-ring species (A1, A2, A3, A4), soot coagulation, and soot oxidation through O2 and OH. A reduced polycyclic aromatic hydrocarbon (PAH) chemistry mechanism was coupled with n-heptane, iso-octane and toluene chemistry mechanisms. The combination of the chemistry mechanisms and the soot model was then validated based on experiments in terms of ignition delay, fundamental premixed flames, SANDIA constant volume chamber spray combustion. The pyrolysis process is also a significant process for soot formation at the conditions of DISI engines. Important species for soot formation from toluene pyrolysis processes were also validated based on experiments, and then coupled with the current n-heptane/iso-octane/toluene/PAH chemistry mechanisms for application to DISI engines. The vaporization of wall films plays a significant role in soot formation and a grid-independent wall film vaporization model was formulated for predicting soot emissions near wall films

Predicted in-cylinder pressure and particle size distributions (PSDs) were compared to available premixed engine experimental studies. Quantitative agreements of in-cylinder particle distributions are also obtained. The improved models were then applied to studies of soot emissions from early- and late-injection strategies in a four-valve single-cylinder gasoline DISI engine, and the trends were consistent with literature or experimental data.