Exhaust gas recirculation (EGR) is widely used in diesel engines to reduce nitrogen oxide emissions. To meet the strict emission regulations, e.g., Real Driving Emissions, the EGR system is required to be used at temperatures lower than the present ones. However, under cool conditions, an adhesive deposit forms on the EGR valve or cooler because of the particulate matter and other components present in the diesel exhaust. This causes sticking of the EGR valve or degradation of the heat-exchange performance, which are serious problems. In this study, the EGR deposit formation mechanism was investigated based on spatially- and time-resolved scanning electron microscopy (SEM) observation. The deposit was formed in a custom-made sample line using real exhaust emitted from a diesel engine. The exhaust including soot was introduced into the sample line for 24 h (maximum duration), and the formed deposit was observed using SEM. The deposit was formed on the surface of the sample line below 300 °C (wall temperature) and had two layers: the deposit formed near the surface of the sample line had hard and dense structures, such as a lacquer (called a hard deposit), and the deposit formed on the hard deposit was powdery and exhibited a “grape-like” shape similar to particulate matter (called a powdery deposit). The amounts of both types of deposits increased in proportion to the duration of the exhaust supply. According to the SEM observations of the hard deposit, powdery deposit, and boundary layer between the positions where the hard and powdery deposits were formed, the hard deposit was formed by taking in the powdery deposit, when the exhaust including soot was supplied.
