Dimitrios N. Tsinoglou

Modeling of automotive fuel cell operation in driving cycles

Mathematical modeling of fuel cells could be a powerful tool for fuel cell stack design optimization and for component optimization for a specific vehicle application. In this paper, a previously published phenomenological model with a valid mechanistic background is further elaborated and embedded in a vehicle simulation software (ADVISOR). This coupling allows investigation of fuel cell operation in driving cycles, taking into account the effect of temperature variation during the driving cycle on the fuel cell operation. Moreover, this methodology enables the investigation of the effects of changing some design parameters of the fuel cell stack, such as maximum stack power, catalyst activity and water concentration in the channels, on the overall performance and fuel consumption of a given vehicle.

Modelling of the selective catalytic NOx reduction in diesel exhaust including ammonia storage

Abstract A mathematical model for the simulation of the selective catalytic reduction of NO x by ammonia in diesel exhaust is presented. The simulation of heat and mass transfer, and flow phenomena in a monolithic catalytic converter is performed by a previously published mathematical model. The chemical phenomena are described by global reactions for the oxidation of ammonia and the reduction of NO x , using Langmuir-Hinshelwood reaction rate expressions. Ammonia storage phenomena are taken into account, based on the adsorption isotherm proposed by Dubinin and Radushkevich. Both the steady state chemical reaction scheme included in the mathematical model and the ammonia adsorption and desorption model are validated against experimental data previously published in the literature. The effect of ammonia storage phenomena under an oscillating NH3/NO x ratio in the feed gas is investigated. The results indicate that for the application examined, ammonia storage plays an important role at intermediate temperatures, where conversion is controlled by reaction kinetics, provided that the ammonia adsorption capacity is sufficiently high.

Effect of perturbations in the exhaust gas composition on three-way catalyst light off

The influence of perturbations in the exhaust gas composition on the operation of three-way catalytic converters (3WCC) has been the subject of many research works. This paper aims to investigate the effect of such transients on the light-off temperature of a commercial 3WCC, by using a dynamic mathematical model for 3WCC simulation. This modeling approach embodies a comprehensive oxygen storage and release submodel into an existing 3WCC quasi-steady model. The dynamic model developed is validated against previously published experimental data, which imply that the presence of transients in the exhaust gas results in an improved low-temperature catalyst performance. Having validated the model, the improved light-off performance is investigated, and attributed primarily to exhaust gas stoichiometry and secondarily to the heat released in the catalyst by the oxidation reactions (self-acceleration). Finally, a parametric study is performed to assess the influence of different patterns of transient exhaust gas composition. The results obtained show that lean composition of the exhaust gas is more favorable during the light-off phase, while frequency and amplitude of the composition oscillation play only a minimal role. This investigation encourages further application of mathematical modeling in areas like lambda control strategy optimization, which were beyond the scope of earlier 3WCC models.

Thermal Response of Close-Coupled Catalysts During Light-Off

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 nonuniform ageing. In fresh catalysts or uniformly aged catalysts, the flow maldistribution is not expected to affect light-off during typical (low flow rate) warm-up conditions. The effect of catalyst insulation and mantle is also studied.

Influence of Pulsating Flow on Close-Coupled Catalyst Performance

Close coupling of automotive three-way catalytic converters is becoming a common practice in order to reduce pollutant emissions during cold start. In such applications, the exhaust gas mass flow may fluctuate, as a function of crankshaft angle. A simplified one-dimensional channel model is developed, assuming that pollutant conversion in the catalyst is mass transfer limited. This model is applied to evaluate the effect of pulsations in catalyst performance, and assess the accuracy of the quasi-steady state approach usually involved in three-way catalytic converter models, when applied to simulate converters under pulsating flow.

MODELLING OF FLOW DISTRIBUTION DURING CATALYTIC CONVERTER LIGHT-OFF

The flow field non-uniformities at the inlet of catalytic converters are considered undesirable for their performance. Computational fluid dynamics (CFD) is a powerful tool for calculating the flow field inside the catalytic converter and optimising design concepts. However, the applicability of CFD for transient simulations is limited by the high CPU demands of this technique. The present study proposes an alternative computational method for the prediction of transient flow fields in axi-symmetric converters time-efficiently. The proposed flow resistance modelling (FRM) method is validated against the results of CFD predictions during a typical warm-up case. The FRM methodology is coupled with an already available transient model for heat transfer and chemical reactions in the catalyst. The effect of flow distribution on pollutant conversion and pressure drop is examined under warm-up and steady state operation.

Modeling the Effect of Flow Pulsations in Close-Coupled Catalytic Converter Light-Off

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.