Abstract
KEYWORDS – GDI Engine, CFD, Soot Modelling, Soot Sectional Method, Star-CD.
ABSTRACT – Modern GDI engines are efficient power platforms, but produce large quantities of ultra-fine soot particles. Fuel mal-distribution and, in some cases, liquid fuel film are commonly addressed as the primary causes of particulate matter formation. Multi-dimensional engine modelling can be used effectively to gain an improved understanding of the in-cylinder processes leading to particulate matter. The work presented here investigates soot mechanisms in a modern wall-guided GDI engine using commercial CFD software Star-CD. Two part-load operating conditions are investigated, 2300 rev/min - 60 Nm, and 2300 rev/min - 120 Nm. The multi-stage semi-empirical Soot Sectional Method is used to simulate the physical and chemical in-cylinder mechanisms leading to soot emissions.
The results of the simulations show better mixture preparation in the high load case, mostly on account of enhanced fuel atomisation and stronger mixing. The lower load case features wider mixture stratification, with a more confined, lower temperature burning zone. In both cases, a strong temperature drop establishes between the hot core and the cylinder walls. Higher levels of oxygen correspond to regions of lower temperature near the walls and vice-versa. This unfavourable arrangement, compounded to the lack of mixture homogeneity, leads to high levels of EVO soot in the lower engine load case.