Modeling of three-way catalyst dynamics during fast transient operation for a CNG heavy-duty engine

In this work, a numerical "quasi-steady" model was developed to simulate the chemical and transport phenomena of a specific Three-Way Catalyst (TWC) for a natural-gas heavy-duty engine. Goal of the present research activity was to investigate the effect of very fast composition transitions of the engine exhaust typical of real world driving operating conditions, as fuel cutoff phases or engine misfire, which produce strong deviations from stoichiometric in Air-to-Fuel (A/F) ratio and characterize catalytic converter efficiency. In fact, according to the literature it is confirmed that the catalyst dynamic behavior differs from the steady-state one due to oxygen storage phenomena. A dedicated experimental campaign has been performed in order to evaluate the catalyst response to a defined ? variation pattern of the engine exhaust stream, thus providing the data necessary for the numerical model validation. A surface reactions kinetic mechanism, concerning CH4, CO, H2 oxidation and NO reduction, has been appropriately calibrated, with a step-by-step procedure, both in steady-state conditions of the engine work plan, at different A/F ratios, and during transient conditions, through cyclical and consecutive transitions of variable frequency between rich and lean phases. The activity also includes a proper calibration of the reactions involving Cerium inside the catalyst, in order to reproduce oxygen storage and release dynamics. Sensitivity analysis and a reactions rate continuous control allowed evaluating the impact of each of them on the exhaust composition in several operating conditions. The proposed model predicts tailpipe conversion/formation of the main chemical species, starting from experimental engine-out data and provides a useful tool for evaluation of the catalyst performance.