Role of Late Soot Oxidation for Low Emission Combustion in a Diffusion-controlled, High-EGR, Heavy Duty Diesel Engine

Soot formation and oxidation are complex and competing processes during diesel combustion. The balance between the two processes and their history determines engine-out soot values. Besides the efforts to lower soot formation with measures to influence the flame lift-off distance for example or to use HCCI-combustion, enhancement of late soot oxidation is of equal importance for low-λ diffusion-controlled low emissions combustion with EGR. The purpose of this study is to investigate soot oxidation in a heavy duty diesel engine by statistical analysis of engine data and in-cylinder endoscopic high speed photography together with CFD simulations with a main focus on large scale in-cylinder gas motion. Results from CFD simulations using a detailed soot model were used to reveal details about the soot oxidation.

A particular objective of the present study was to investigate the importance of enhancing soot oxidation after End of Injection (EOI) when temperature and NOx formation rapidly decreases. Geometrical measures to control flame propagation and different flame interactions were investigated. Such measures contribute to conserve available kinetic energy until late in the combustion period in an efficient way. Based on this combustion strategy it is possible to reach near zero engine-out soot emissions.