Development of a 1D-CFD model of an aftertreatment system for a hydrogen fueled engine

Nowadays the reduction of the transportation noxious emissions plays a crucial role in the development of new traction systems and new vehicles. All the OEMs are switching from pure thermal powertrains to hybrid or pure electric ones in order to comply with the always more stringent regulations developed with the purpose of emissions reduction. While for passenger cars the industry is moving towards electric powertrains, for heavy duty vehicles the electrification has more limitations given by the energy storage, the increment of the vehicle weight and the recharging time and infrastructure. For these reasons, the hydrogen engine represents a promising technology to achieve immediate improvements in terms of the decarbonization of the transportation sector especially in the heavy duty field. Consequently, the aftertreatment systems developed initially for diesel engines must be adapted to this new technology considering the differences on the exhaust gas composition. In this framework, this thesis work has the purpose of developing and analyzing a 1D simulation model in GT-Suite with the aim of reducing the tailpipe emissions of a hydrogen engine applied to a hybrid powertrain for a 12 m urban bus. First, an aftertreatment system composed by a SCR (Selective Catalytic Reduction) catalyst only has been analyzed through numerical simulation, being the emissions of the hydrogen engine mainly composed by NOx. Furthermore, the influence of two different chemical compositions of the catalyst (Cu-Zeolite and Fe-Zeolite) on the performances of the aftertreatment system has been evaluated taking into account the total conversion efficiency, the ammonia slip, and the urea consumption. Moreover, the influence on the catalyst performance of geometrical parameters such as the cell density, of the active site density and of the urea injected quantity has been analyzed. Finally, the influence of an OC (Oxidation Catalyst) placed upstream the SCR with the aim to increase the NO2/NOx ratio and increase the SCR conversion efficiency at low temperatures has been evaluated. In conclusion, for this case study, the aftertreatment system composed by the Fe-Zeolite SCR only and the one featuring a small OC have similar performances, while the presence of a bigger OC degrades the performances. The presence of an OC with a higher volume, in fact, causes issues for the SCR light-off and, as a consequence, the positive effect given by a favorable NO2/NOx ratio is not fully exploited. However, it should be pointed out that these results are determined mainly by the composition of the exhaust gases and in particular by the very low levels of engine out emissions NOx produced by the ultra-lean engine calibration. Therefore in future works the model should be tested with a different engine calibration producing higher NOx engine out emissions, as well as considering also the presence of CO and HC produced by the combustion of lubricant oil in order to have a more complete and robust assessment of the aftertreatment system potential for achieving close to zero emission levels for hydrogen fueled engines.