Past research has shown that post injections have the potential to reduce Diesel engine exhaust PM concentration without any significant influence in NOx emissions. However, an accurate, widely applicable rule of how to parameterize a post injection such that it provides a maximum reduction of PM emissions does not exist. Moreover, the underlying mechanisms are not thoroughly understood. In past research, the underlying mechanisms have been investigated in engine experiments, in constant volume chambers and also using detailed 3D CFD-CMC simulations. It has been observed that soot reduction due to a post injection is mainly due to two reasons: increased turbulence from the post injection during soot oxidation and lower soot formation due to lower amount of fuel in the main combustion at similar load conditions. Those studies do not show a significant temperature rise caused by the post injection.
Previous investigations led to the conclusion that the effectiveness of a post injection increases with the amount of oxygen transported from the post injection into the soot cloud of the main injection. Hence, the potential of interaction of the two soot clouds as well as the local oxygen availability in-crease the soot reduction through post injection. This creates a trade-off, since the interaction potential usually increases with lower (global) oxygen availability. The current work provides new 3D simulations in an engine and 2D visualisation of soot evolutions recorded in an optically high accessible constant volume chamber, with focus on the trade-off between oxygen availability and soot cloud interaction potential. Moreover, the conclusions of this work have been applied in a standard modern common rail passenger car engine. The engine is equipped with an optical light probe to obtain the in-cylinder soot evolution.
The 3D simulations highlight the importance of the interaction between the soot clouds and show the transport of oxygen due to the post injection into the soot cloud of the main injection. The reduction of ambient oxygen concentration increases the oxidation rate up to a certain limit due to higher degree of soot cloud interaction. The results of the constant volume chamber confirm that soot reduction is limited at a certain oxygen availability limit. The engine experiments show the trade-off between oxygen availability and the interaction potential and thus confirm the results obtained from the simulation and the generic test rig.