Development and calibration of a stand-alone Selective Catalytic Reduction model in the GT-SUITE environment

In today's automotive industry, ensuring that engines and aftertreatment systems operate at maximum efficiency under all working conditions is of paramount importance. This is not only essential for complying with stringent emission regulations but also crucial for addressing global warming and ensuring an adequate level of air quality. Diesel engines have substantial potential for carbon dioxide reduction due to their higher efficiency compared to spark ignition engines. However, it is crucial to suppress NOx emissions and particulate matter. Regarding NOx emissions, a dedicated after-treatment device must be employed, as the high oxidizing conditions in which diesel engines operate preclude the effective use of the three-way catalyst. In this context, Selective Catalytic Reduction (SCR) is one of the most widely adopted solutions. The first part of this work focuses on a detailed analysis of the operating principle of this system and on the influence of the ammonia-to-NOx ratio (ANR) on its performance. In SCRs, a specific fluid is injected into a catalyst, from which ammonia originates and reacts with NOx to form nitrogen and water. The ANR is defined as the ratio between the Ammonia concentration at the inlet of the SCR and the NOx concentration at the engine outlet. While a stoichiometric ratio might seem optimal based exclusively on the reactions involved in the catalyst, the literature review reveals that the SCR performance can be optimized if the ANR is adjusted in accordance with the temperature. This strategy aims at finding a good compromise between conversion efficiency and ammonia slip, without neglecting the urea economy. Subsequently, a preliminary analysis of the ANR selection as a function of the temperature has been conducted. To lay the foundations of a potential strategy for selecting the optimal value of ANR, the definition of a multi-objective cost function, which considers both the SCR conversion efficiency and the ammonia slip, has been implemented. Moreover, the cost function allows assigning different weights to the objectives, which can be selected properly on the basis of the specific application. Thereby, by calibrating the weights of the cost function, it is possible to perform a preliminary prediction of a potentially suitable strategy for different scenarios. Finally, the focus shifts to the development of a stand-alone SCR digital model in the GT-SUITE environment. The advantages of using a digital model instead of laboratory tests on real SCRs lie in the considerable cost and time savings. The main objective was first to identify the key parameters and then to calibrate them to obtain a model that can predict realistic results. To achieve this, a combination of experimental data acquired at the laboratory test bench and data derived from literature research was used to implement the calibration and compare the model results with the experimental ones. Ultimately, the employed calibration methodology demonstrates that it is possible to develop a model capable of predicting the trends observed in the experimental data.