Development of a Dedicated CNG Three-Way Catalyst Model in 1-D Simulation Platforms

A growing interest towards heavy-duty engines powered with NG, dictated by stringent regulations in terms of emissions, has made it essential to study a specific Three-Way Catalyst (TWC). Oxygen storage phenomena characterize the catalytic converter efficiency under real world driving operating conditions and, consequently, during strong dynamics in Air-to-Fuel ratio (AFR). A numerical "quasi-steady" model has been set-up to simulate the chemical process inside the reactor. A dedicated experimental campaign has been performed in order to evaluate the catalyst response to a defined ? variation, thus providing the data necessary for the numerical model validation. In fact, goal of the present research activity was to investigate the effect of very fast composition transitions of the engine exhaust typical of the mentioned driving conditions (including fuel cutoffs etc.) on the catalyst performance and on related emissions. A surface reactions kinetic mechanism, representing CH4, CO, H2 oxidation and NO reduction, has been appropriately calibrated in steady-state operation, using a stepby- step procedure all over the engine operating conditions at different AFRs. Then transient conditions were numerically reproduced, through cyclical and consecutive transitions of variable frequency between rich and lean phases. The model includes a proper calibration of the reactions involving Cerium inside the catalyst, in order to reproduce oxygen storage and oxygen release dynamics. Monitoring the reaction rates of the adopted mechanism permitted to evaluate their impact on the exhaust stream 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.