Elucidation of exhaust gas recirculation deposit formation mechanism based on chemical analysis of components

Abstract

To comply with strict emission regulations, vehicles are equipped with exhaust gas recirculation (EGR) systems that control nitrogen oxide emissions by returning part of the exhaust gas to the intake air. However, since deposits form in the EGR piping and valves, leading to pipe clogging, valve sticking, and in the worst case, failure of these components, there is a need to clarify the mechanism of deposit formation to predict and reduce deposit accumulation. While most of the previous studies have focused on the powdery deposits that clog the EGR cooler, this study aims to elucidate the formation mechanism of both the lacquer-like hard deposits and wet soft deposits that cause EGR valve sticking by quantitative and molecular-level analyses. In these experiments, a portion of the exhaust gas from an actual diesel engine without soot removed by diesel particulate filter (DPF) was used. Deposits were continuously generated on a simulated EGR line with a downstream temperature gradient from the engine outlet, while measuring the inner wall temperature. The collected deposits were subjected to comprehensive chemical analysis. The analysis showed that 75 % of the solvent-soluble deposit components were polycyclic aromatic hydrocarbons (PAHs), which were the main component of the hard and soft deposits. Most of the PAHs in the hard deposits that formed at a high inner wall temperature were C17 or higher. The hard deposit with an inner wall temperature of 137 °C contained 0.016 mg/cm2 of 6-ring benzo[ghi]perylene and 0.26 × 10-3 mg/cm2 of 4-ring pyrene. On the other hand, the soft deposit is formed at an inner wall temperature of 80 °C, contained 0.022 mg/cm2 of pyrene, which is significantly more than in the hard deposit. This shows that PAHs in the exhaust gas condense when the gas temperature reaches their dew points, and that the deposits formed at higher inner wall temperatures contain more PAHs with higher boiling points. High-molecular weight aromatic compounds with oxygen and nitrogen functional groups were detected in the solvent-insoluble deposit components. These compounds were found to be derived from the polymerization of oxygenated and nitrogenated PAHs through dehydration condensation and cross-linking reactions.