CFD Investigation of the Impact of Electrical Heating on the Light-off of a Diesel Oxidation Catalyst

In the last years, as a response to the more and more restrictive emission legislation, new devices (SRC, DOC, NOx-trap, DPF) have been progressively introduced as standard components of modern after-treatment system for Diesel engines. In addition, the adoption of electrical heating is nowadays regarded with interest as an effective solution to promote the light-off of the catalyst at low temperature, especially at the start-up of the engine and during the low load operation of the engine typical of the urban drive. In this work, a state-of-the-art 48 V electrical heated catalyst is considered, in order to investigate its effect in increasing the abatement efficiency of a standard DOC. The electrical heating device considered is based on a metallic support, arranged in a spiral layout, and it is heated by the Joule effect due to the passage of the electrical current. As a result of the spiral arrangement, the distribution of the heat source on the heating section is not uniform, determining a certain spatial distribution of the temperature of the gas entering the DOC section. This has also an influence on the pollutant conversion, both in term of light-off time and overall conversion. In order to simulate the after-treatment system, a suitable CFD framework has been implemented on the basis of the open-source OpenFOAM code. In particular, it is based on a multi-region approach, where overlapping meshes, describing fluid and solid regions, are employed in order to model the presence of porous substrates. Specific models are implemented in order to couple fluid and solid regions in terms of heat-transfer and mass-transfer. Catalytic reaction model is introduced in order to describe the chemical surface reactions occurring on the washcoat of the porous substrate. The model is firstly validated resorting to experimental data. Then, it is applied for the investigation of the effects of the electrical heating on the pollutant abatement, with particular focus on the effects of the non-uniform temperature distribution related to different layouts of the heating spirals. The study points out the benefits related to the adoption of the electrical heating in terms of reduction of the overall pollutant emissions over the RDE cycle. Moreover, the effects of the non-uniform heating is investigated, showing a certain role in promoting the light-off of the reactions as a consequence of the formation of hot spots in the catalyst.