Development and application of a channel-scale exhaust after-treatment model

Abstract

This thesis presents the development and application of a computational model for optimisation and investigation of rare behaviours of exhaust after-treatment systems. Firstly, the performance of the model is critically assessed for various after-treatment devices against literature data, including a diesel oxidation catalyst (DOC), a selective catalytic reduction (SCR) device and a diesel particulate filter (DPF). The model is then applied to study the impact of the configuration of a multi-device after-treatment system. An after-treatment design case study for a heavy-duty diesel engine is performed. The case study showed that the DOC-DPF-SCR layout can lead to better fuel economy in the engine than the DOC-SCR-DPF layout. The commonly claimed promotional effect of the DOC on the SCR performance in the SCR-front layout is not observed in the case study. A flux analysis is performed which explains why the DOC-SCR-DPF configuration is less preferable for the particular SCR catalyst employed. Transient particulate emission from an active regenerating DPF is investigated using the model. The filtration model is extended based on experimentally observed non-uniform combustion of the soot cake layer and a reduction in filtration efficiency of the filter wall at high temperature. The extended model is shown to be able to better describe the elevated particulate emission during active regeneration in both the timing of particle breakthrough and the final steady filtration efficiency of the hot regenerated DPF. The viability of thermal treatment as an ash management strategy for ash-contaminated particulate filters is investigated. The model is applied to simulate the deposition of soot and ash particles in order to estimate the spatial distribution of ash deposits within the particulate filters. The introduction of phenomenological sintering and cracking models for ash deposits shows good agreement between model predicted and experimentally measured pressure drop of thermally treated filters. Crack formation in the ash layer is identified to be the most important phenomenon for a successful thermal treatment.