EXPERIMENTAL AND NUMERICAL INVESTIGATION OF A DIESEL AFTERTREATMENT SYSTEM

Automotive diesel engines, thanks to their high efficiency, play a key role in the future global CO2 emission reduction. However, the upcoming tighter regulations with the aim of reduction of the harmful pollutant emissions require the adoption of more complex after-treatment systems, where the integration between different components is mandatory in order to minimize the overall drawbacks and maximize the system efficiency. In particular, in this work a Diesel Particulate Filter (DPF) and a Diesel Oxidation Catalyst (DOC) were experimentally and numerically investigated, considering both the integration with other after-treatment components and the impact of the usage of different fuel compositions. Since the properties of the filter substrate material play a fundamental role in determining the optimal soot loading level to be reached before DPF regeneration, three different filter substrate materials (Silicon Carbide, Aluminum Titanate and Cordierite) were investigated in this work, considering different driving conditions, after-treatment layouts and regeneration strategies. In the first step of the research, an experimental investigation on the three different substrates over the New European Driving Cycle (NEDC) was performed. The data obtained from experiments were then used for the calibration and the validation of a one dimensional fluid-dynamic engine and after-treatment simulation model. Afterwards, the model was used to predict the vehicle fuel consumption increments as a function of the exhaust back pressure due to the soot loading for different driving cycles. The results showed that appreciable fuel consumption increments could be noticed only in particular driving conditions, and, as a consequence, in most of the cases the optimal filter regeneration strategy corresponds to the highest soot loading that still ensures the component safety even in case of uncontrolled regeneration events. The diesel engine commercialization in emerging markets (like India and Asia) and the contemporary adoption of more stringent emission regulation set a further issue for the diesel after-treatment system performance, since the low diesel fuel quality enhances the risk of exceeding the emission limits. One of the most relevant characteristics of the low quality diesel fuel is the high sulfur content which has an adverse influence on emissions. Utilization of high sulfur fuel can cause deactivation of diesel catalyst and as a result higher amount of pollutants are observed at the tailpipe. For this reason it is crucial to understand and mitigate sulfur impact and the extent of catalyst efficiency recovery through de-sulfation processes in order to increase the robustness of the EU5/6 diesel after-treatment systems also for markets with high sulfur fuel. Considering the abovementioned issues, the impact of high sulfur fuel on the Diesel Particulate Filter and Diesel Oxidation Catalyst performance throughout different stages of New European Driving Cycle (NEDC) was experimentally tested on real size engine tests. In order to assess the impact of sulfur poisoning, a specific poisoning procedure was adopted which resulted in different sulfur poisoning levels. The impact of different space velocities on degreened, poisoned and de-sulfated system was examined and compared. In addition, the ability of recovering the performance of after-treatment system after regeneration through a proper de-sulfation strategy was evaluated with respect to fresh and degreened catalyst. To come to the point, DOC showed a continuous loss of performance after each poisoning procedure, while DPF reached a steady state after a certain level of poisoning. DPF seemed to be capable of recovering the efficiency gap highlighted over the DOC only when lightly sulfur poisoned systems were considered. Moreover, light-off temperature was not affected by Space Velocity (SV) in the degreened catalyst, while for de-sulfated and poisoned catalyst the opposite was found. Besides, changing the SV revealed various impacts on the HC light-off curve.