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Evaluation of Mechanism for EGR Deposit Formation Based on Spatially- and Time-Resolved Scanning-Electron-Microscope Observation
ISSN: 0148-7191, e-ISSN: 2688-3627
To be published on September 15, 2020 by SAE International in United States
Exhaust gas recirculation (EGR) is widely used in diesel engines to reduce nitrogen oxide emissions. To meet strict emission regulations such as real driving emission, the EGR system has to be used under temperature conditions lower than the present ones. However, an adhesive deposit forms on the EGR valve or cooler under such cool conditions because of the particulate matters and other components present in the diesel exhaust, which are serious problems. In this study, the formation mechanism of EGR deposit was investigated based on spatially- and time-resolved scanning electron microscope (SEM) observation. The deposit was formed in a custom-made sample line using real exhaust emitted from a diesel engine. The exhaust was introduced into the sample line for 24 h (maximum duration) and the formed deposit was observed using the SEM. The deposit was formed on the surface of the sample line below 500 K and had two layers: deposit formed near the surface of the sample line had hard and dense structures like a lacquer (called hard deposit), and deposit formed on the hard deposit was powdery and exhibited porosity similar to a particulate matter (called powdery deposit). The amount of both types of deposit increased in proportion to the duration of the exhaust supply. Based on the SEM observation of the hard deposit, powdery deposit, and boundary layer between the positions where the hard and powdery deposits were formed, it was found that the hard deposit was formed by collecting the powdery deposit.