Mechanism of White Smoke Generation Derived from Hydrocarbons Accumulations on Diesel Oxidation Catalyst

White smoke emission is observed at the tailpipe of diesel vehicles when unburned hydrocarbons (HCs) are adsorbed on a diesel oxidation catalyst (DOC) under low exhaust gas temperature. The purpose of this study is to gain a better understanding of white smoke emission derived from HCs, and to reduce emission levels. First, the components of HCs and the particle size distribution of white smoke emission were analyzed. It was clarified that semi-volatile organic compounds (SVOC) and water are condensed around soluble organic fraction and the order of particle size in white smoke is submicron scale. Additionally, the correlation between the behavior of white smoke emission and the amount/quality of HCs adsorbed on a DOC were investigated by examining the change of zeolite content in the DOC. It was found that the heavy HCs ratio in adsorbed HCs on DOC increases with a decrease in zeolite content when DOC inlet gas temperature is 120 °C. Based on these experimental results and considerations, the mechanisms of white smoke generation derived from HCs were assumed and confirmation testing was conducted using by DOCs having different specific surface area (SSA) alumina. As a result, it was found that adopting small SSA alumina to DOC achieves a breakthrough in the trade-off between white smoke emission, HCs, CO conversion performance.