Search Results

Even though the 3-way catalyst chemistry has been studied extensively in the literature, some performance aspects of practical relevance have not been fully explained. It is believed that the Oxygen Storage Capacity function of 3-way catalytic components dominates the behavior during stoichiometry transitions from lean to rich mode and vice versa whereas a number of mathematical models have been proposed to describe the dynamics of pollutant conversion. Previous studies have suggested a strong impact of Sulfur on the pollutant conversion after a lean to rich transition, which has not been adequately explained and modelled. Lean to rich transitions are highly relevant to catalyst ‘purging’ needed after exposure to high O2 levels (e.g. after fuel cut-offs). This work presents engine test measurements with an engine-aged catalyst that highlight the negative impact of Sulfur on pollutant conversion after a lean to rich transition.

Modern ‘DOC-cDPF’ systems for diesel exhaust are employing Pt-, Pd- as well as Pt/Pd alloy- based coatings to ensure high conversion efficiency of CO, HC even at low temperatures. Depending on the target application, these coatings should be also optimized towards NO2 generation which is involved in low temperature soot oxidation as well as in SCR-based deNOx. Zeolite materials are also frequently used to control cold-start HC emissions. Considering the wide variety of vehicles, engines and emission targets, there is no single optimum coating technology. The main target is therefore to maximize synergies rather than to optimize single components. At the same time, the system designer has nowadays a wide range of technologies to choose from, including PGM alloyed combinations (Pt/Pd), multiple layers and zones applicable to both DOCs and DPFs.

In real-world driving, the exhaust gas conditions in the particulate filter may induce incomplete filter regenerations. The implications of such partial regenerations are examined in this paper in terms of pressure drop and filter thermal loading. The methodology followed is based on a 2-D simulation model of the regeneration process. The model is initially fine-tuned and validated based on experimental results from engine bench testing. The validated model is subsequently employed to simulate a series of regenerations starting from different possible initial soot distribution patterns. The results are evaluated based on the calculated maximum thermal gradient in the filter that would produce the critical thermal stress for filter structural integrity. It is shown that the filter thermal loading can be significantly higher in case of initially non-uniform soot distribution.

A case study of a close-coupled catalyst subjected to exhaust gas conditions typical for a modern engine warm-up phase is studied using a time-efficient 2-dimensional modeling approach. The flow distribution at catalyst inlet is affected by the downstream flow resistance, which is in turn a function of catalyst temperature field. Unlike traditional CFD approaches, the presented model focuses on this interesting coupling between the problems of flow distribution and catalyst thermal response. The results are expressed in terms of time-dependent velocity and temperature distribution as well as conversion efficiency. After a basic understanding of the phenomena, a parametric analysis is performed to assess the significance of various design parameters affecting the cold start performance of close-coupled catalysts. It is shown that the detrimental effect of flow uniformity on light-off is associated to the non-uniform ageing.

The pressure for compact and efficient deNO systems has led to increased interest of incorporating SCR coatings in the DPF walls. This technology could be very attractive especially if high amounts of washcoat loadings could be impregnated in the DPF porous walls, which is only possible with high porosity filters. To counterbalance the filtration and backpressure drawbacks from such high porosity applications, the layered wall technology has already been proposed towards minimizing soot penetration in the wall and maximizing filtration efficiency. In order to deal with the understanding of the complex interactions in such advanced systems and assist their design optimization, this paper presents an advanced modeling framework and selected results from simulation studies trying to illustrate the governing phenomena affecting deNO performance and passive DPF regeneration in the above combined systems.

The successful design and especially the control of the SCR system is a challenging process that can be supported by the application of simulation tools. As a first step, we employ physico-chemically informed ‘off-line’ models that are calibrated with the help of targeted small- and full-scale tests. Despite their high level of sophistication, this SCR model is able to be integrated in a control-oriented simulation software platform and connected to other powertrain simulation blocks. The target is to use this simulation platform as a virtual environment for the development and optimization of SCR control strategies. The above process is demonstrated in the case of a passenger car SCR. The model is calibrated at both fresh and aged catalyst condition and validated using experimental data from the engine bench under a wide variety of operating conditions. Next, the calibrated model was coupled with embedded control models, developed for Euro 6 passenger car powertrains.

In close-coupled catalytic converter applications, the exhaust gas mass flow may present fluctuations with a timescale in the order of milliseconds, as a result of periodic valve operation. Such flow pulsations are likely to affect catalytic converter performance, due to mass transfer limitations. A fully transient channel model is developed, to study pollutant conversion under pulsating flow conditions. This model is applied to evaluate the effect of pulsations in catalyst conversion. Firstly, the effect of flow pulsations during the warmed-up operation is studied. Then, we focus on the effect of pulsations during the light-off phase, during which significant temperature and therefore activity gradients are observed in the monolith. The new model is also used to assess the accuracy of the traditional “quasi-steady state” approach, when applied to simulate converters under pulsating flow.

Hydrocarbon traps for gasoline engines are promising candidates for cold start emission control, provided that their design is based on a “systems approach”. In this paper, an existing CAE methodology for exhaust after-treatment is expanded to include HC trap technology. The flow, heat transfer and chemical kinetics in a typical complex system, comprising a “barrel type” adsorber and two conventional catalysts are studied. A mathematical model is developed and applied for the computation of the flow and pressure distribution, as well as transient heat transfer in the barrel type adsorber. A physically relevant model is used to simulate HC adsorption desorption on the adsorbing material. The model is used in combination with an existing 2-d 3-way catalyst model to simulate different HC trap concepts. The aim is to understand and quantify the particular thermal response and HC retention behavior of hydrocarbon adsorber systems.