Birth and Death of Soot Deposits and the Mechanism of Particle Emissions During Regeneration in Diesel

Athanasios G.  Konstandopoulos, APT Lab, CPERI/CERTH

The structure and properties of soot deposits in Diesel Particulate Filter (DPF) affects significantly the DPF pressure drop, hence the fuel penalty imposed on the vehicle.  If one tries to describe the soot layer grown on by a uniform density (or equivalently porosity), surface area and permeability, and try to determine these parameters from experiments he would arrive at widely varying values. We have shown (Konstandopoulos et al. 2002; 2007) that during DPF loading the microstructure of the soot cake is determined by the convective-diffusive transport of the soot aggregates towards the deposit and it was also demonstrated that soot cake packing density and permeability are related to the local value of the dimensionless mass transfer Peclet number. In addition as shown here these parameters can be related to the fractal-like morphology of the aggregates.

During regeneration events the soot deposit reacts in a generally inhomogeneous fashion and frequently one observes increased particle emission during these events. While emissions of volatile particles during regeneration events of Diesel Particulate Filters (DPFs) is commonly understood to be caused by the thermal desorption of organic compounds that are condensed on the accumulated solid soot particles, the emissions of very small solid particles during regeneration events is less understood.  Filtration theory and testing experience over many years indicates that small soot nanoparticles should and are very efficiently collected by DPFs. What is then the mechanism by which very small solid particles are emitted during some regeneration events? In the present work we provide a fundamental mechanism responsible for the emission of small solid nanoparticles during DPF regeneration and investigate measures that can be employed to prevent such emissions.