A chemical kinetic modelling study of the mechanism of soot reduction by oxygenated additives in diesel engines

Abstract

Oxygenated fuel additives are effective in reducing soot emissions and breaking the soot/NO_x tradeoff in diesel engines. The extent to which molecular structure of the oxygenate plays a role in soot reduction has remained unclear and controversial. It has been suggested in the literature that the rapid breakdown of oxygenates to carbon monoxide (CO) and carbon dioxide (CO₂), and subsequent interaction between CO and CO₂ with the radical pool, may play a key role. Here a constant-pressure homogeneous reactor model, SENKIN, has been used to simulate the effect of small amounts of CO and CO₂ on the formation of polycyclic aromatic hydrocarbons, as surrogates for soot, and other key species such as acetylene (C₂H₂) using three different detailed chemical mechanisms. The addition of O₂ was also studied as a reference case. It is found that polycyclic aromatic hydrocarbon (PAH) and C₂H₂ decrease with CO addition, and increase slightly with the addition of CO₂, regardless of the mechanism used. These results suggest that CO produced by oxygenated additives in diesel fuel is able to effect reduction in PAH as suggested in the literature, but does not confirm a similar effect from CO₂.