The main objective of the present research activity was to investigate the effect of very fast composition transitions of the engine exhaust typical in real-world driving operating conditions, as fuel cutoff phases or engine misfire, on the aftertreatment devices, which are generally very sensitive to these changes. This phenomenon is particularly evident when dealing with engines powered by natural gas, which requires the use of a three-way catalyst (TWC). Indeed, some deviations from the stoichiometric lambda value can interfere with the catalytic converter efficiency. In this work, a numerical “quasi-steady” model was developed to simulate the chemical and transport phenomena of a specific TWC for a compressed natural gas (CNG) heavy-duty engine. A dedicated experimental campaign was performed in order to evaluate the catalyst response to a defined λ variation pattern of the engine exhaust stream, thus providing the data necessary for the numerical model validation. Tests were carried out to reproduce oxygen storage phenomena that make catalyst behavior different from the classic steady-state operating conditions. A surface reaction kinetic mechanism concerning CH₄, CO, H₂, oxidation and NO reduction has been appropriately calibrated at different λ values with a step-by-step procedure, both in steady-state conditions of the engine work plan 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 continuous control of the reaction rate 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 to evaluate the catalyst’s performance.

This paper describes some strategies to deal with the arduous challenge of reducing emissions from the transport sector. Two different approaches in particle emissions reduction from natural gas (NG) heavy duty (HD) engines were evaluated. The focus was on reducing the ultra-fine sub 23 nm particles, a key aspect in the vehicles’ impact on human health and environment. To this end, an experimental research activity was carried out on a NG HD engine that was EURO VI regulation compliant. Lubricant oils characterized by different base compositions and ash contents were compared to provide a preferred path to develop formulations. The performed activity on world harmonized transient cycles (WHTCs) have demonstrated a high reduction potential (≈70%) that is reachable by acting on the lube formulation. A CNG particle filter (CPF), derived from the diesel and gasoline engines technology, was fully characterized in terms of its filtration efficiency. Three different types of tests were carried out: steady state, WHTCs, and several idle-to-load step maneuvers. The CPF was highly efficient in reducing solid particles over 10 nm diameter in all the different tests. During WHTCs, the mean abatement efficiency was about 85%. Both technologies provide interesting insights to make NG HD engines compliant with the upcoming Euro VII regulation.

Research activities in the development of reliable computational models for aftertreatment systems are constantly increasing in the automotive field. These investigations are essential in order to get a complete understanding of the main catalytic processes which clearly have a great impact on tailpipe emissions. In this work, a 1D chemical reaction model to simulate the catalytic activity of a Pd/Rh Three-Way Catalyst (TWC) for a Natural Gas heavy-duty engine is presented. An extensive database of tests carried out with the use of a Synthetic Gas Bench (SGB) has been collected to investigate the methane abatement pathways, linked to the lambda variation and oxide formation on palladium surface. Specific steady-state tests have shown a dynamics of the methane conversion even at fixed λ and temperature conditions, essentially due to the Pd/PdO ratio. Furthermore, combining the results of such test with dedicated Rich-Lean λ transitions it has been demonstrated that the presence of NO reduces the rate of the methane oxidation reaction. Given the high reliability of the experimental data and the possibility of managing the chemical composition of the gas entering the catalyst, important aspects related to the NH₃ formation were analyzed. In the proposed kinetic scheme, NH₃ decomposition phenomena are also present at high temperature due to the presence of Rh in the catalyst. Reactions involving Cerium for oxygen storage and release characterization are included in the proposed model in addition to the surface reaction mechanism, reasonably determining the TWC conversion efficiency of the main species.

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 λ 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 step-by-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.