Oxidation Characteristics of Gasoline Direct-injection (GDI) Engine Soot – Catalytic Effects of Ash
Seungmok Choi, Argonne National Laboratory
Gasoline direct-injection (GDI) engines have been reported to produce more particulate matters (PM) than do conventional port-fuel-injection (PFI) gasoline spark-ignition engines, especially during engine warm-up and transient conditions. Gasoline particulate filter (GPF) systems have been considered for the GDI engines, which, in turn, require a comprehensive understanding of GDI soot oxidation kinetics. In this study, the composition and oxidation characteristics of GDI engine PM were investigated for various engine operating conditions and sampling positions (engine-out and post three-way catalyst (TWC)). The amount of ash in the GDI engine soot was an order of magnitude higher than that in conventional diesel engines, and the ash fraction increased as the total soot mass emissions decreased. The TWC reduced the organic fractions and at the same time significantly increased the ash fraction in GDI soot. EDXS analysis showed that the ash is originated from the lube oil consumption as well as detachment of supporting materials of the TWC. The oxidation characteristics of the GDI soot were considerably different from the model soot (carbon black), showing a three-staged oxidation which has faster oxidation rates at the initial and the last parts by the effects of soluble organics and ash. The oxidation kinetics of the GDI soot did not obey the typical Arrhenius correlation; therefore, a modified kinetic correlation has been developed, in which the effects of SOF and ash can be taken into account. The oxidation reactivity of the GDI soot strongly depended on the ash fraction; faster soot oxidation by a factor of two as 1 % increase of the ash fraction, and it is clear that the ash plays a catalytic role that enhances the GDI soot oxidation reactivity.