G. C. Koltsakis

Development and application range of mathematical models for 3-way catalytic converters

Abstract The need for reliable 3-way catalytic converter modeling in the design of demanding exhaust systems for low-emitting vehicles has been widely recognised. Although a number of related models have been presented in the literature, the efficient performance in actual 3-way applications requires further development and validation. The major difficulties posed in such modeling efforts arise from the complexities in the reaction schemes and the respective rate expressions for the multitude of currently used catalytic formulations. This paper presents a two-dimensional catalytic converter model, featuring a number of innovations regarding the catalyst transient behaviour, the reaction kinetics and the solution procedure. The oxygen storage submodel presented is capable of accounting for the redox and temperature dependence of the oxygen availability under transient operation. The redox sensitivity of the reaction scheme gives a clearer insight in the ‘lambda-window’ behavior of 3-way catalysts. The application range of the model and the expected accuracy levels in the most common engineering problems are discussed. It is concluded, that although the task of predicting emissions over random driving scenarios is quite demanding in both chemical kinetics and inlet conditions data, most optimization applications may be sufficiently handled with existing kinetic expression information.

Transient heat transfer modelling in automotive exhaust systems

Abstract Transient heat transfer computations in automotive exhaust systems are increasingly employed in the design and optimization phases. The complex geometry of the exhaust line and the special flow conditions complicate the problem of accurately estimating several important heat transfer parameters. This paper initially summarizes the current status of knowledge regarding heat transfer phenomena in automotive exhaust systems. A comprehensive transient computer model covering all exhaust piping configurations (single wall, double wall with air gap or insulation) is presented. A novel solution procedure is proposed, resulting in significant savings in processing time. Two-dimensional heat transfer in connecting flanges is also accounted for. The model is validated with the help of full-scale measurements on vehicles. Examples are presented, illustrating the application of the model in the comparative assessment of different exhaust configurations. In conjunction with existing models, which simulate the operation of three-way catalytic converters and of other exhaust gas after-treatment devices, the model can be integrated in a CAE (computer aided engineering) package for the support of exhaust system design optimization.

THREE-WAY CATALYTIC CONVERTER MODELING AND APPLICATIONS

Abstract The advent of stricter U.S. and European exhaust emissions regulations has increased the need for reliable 3-way catalytic converter models supporting the design of demanding exhaust systems for low-emitting vehicles. Although a number of tunable models have been presented in the literature, their efficient performance in actual 3-way applicaions requires further development and validation. The major difficulties posed in such modeling efforts arise from the complexities in the reaction schemes and the respective rate expressions for the multitude of currently used catalyst formulations. This paper addresses the details of tuning and real world application of a two-dimensional catalytic converter model, which accounts for the HC (hydrocarbons) and CO oxidation, as well as NO reduction reactions. The model features a number of innovations regarding the catalyst transient behaviour modelling and the reaction kinetics scheme. The advanced oxygen storage submodel presented is capable of accounting fo...

Computer aided engineering in diesel exhaust aftertreatment systems design

Abstract Computer aided engineering (CAE) methodologies are increasingly being applied to assist the design of spark-ignition (SI) engine exhaust aftertreatment systems in view of the stage III and IV emissions standards. Following this trend, the design of diesel exhaust aftertreatment systems is receiving more attention owing to the capabilities of recently developed mathematical models. The design of diesel exhaust systems must cope with three major aftertreatment categories: diesel oxidation catalysts, diesel particulate filters and de-NOx catalytic converters. An integrated CAE methodology that could assist the design of all these classes of systems is described in this paper. It employs the following computational tools: a computer code for modelling transient exhaust system heat transfer, a computer code for modelling the transient operation of a diesel oxidation or a de-NOx catalytic converter, a database containing chemical kinetics data for a variety of oxidation and de-NOx catalyst formulations and a computer code for modelling the loading and regeneration behaviour of a wall-flow filter, assisted by catalytic fuel additives. Application of the CAE methodology, which helps the exhaust aftertreatment system design engineer to meet the future emissions standards, is highlighted by referring to a number of representative case studies.

MATHEMATICAL MODELLING OF PRECIOUS METALS CATALYTIC CONVERTERS FOR DIESEL NOX REDUCTION

Abstract Precious metal catalysts for NO x reduction in lean diesel engine exhaust conditions are characterized by a narrow temperature range of efficient operation and require high availability of reducing species in significant concentration. Consequently, there exists a large optimization potential in the design and control of lean-NO x catalytic conversion systems. A mathematical model of the transport and chemical phenomena in platinum-based lean-NO x catalysts was formulated, based on the experience with analogous models for gasoline three-way catalysts. A simplified four-reaction scheme is employed, considering the oxidation of CO, H2 and hydrocarbons (HCs), as well as the reaction between NO x and HCs. Results are compared with previously published laboratory and engine data in order to assess the capacity of this approach in representing real-world behaviour of Pt-based lean-NO x catalysts. Initial results illustrate the power and flexibility of the model, which is able to predict the NO x conversion characteristics in model gas tests as well as in full-scale engine tests with reasonable accuracy.

Some Empirical Observations on Diesel Particulate Filter Modeling and Comparison Between Simulations and Experiments

Comparisons between 1D simulations and experiments on a mini scale SiC filter are presented. First of all, experiments with regeneration for different loading mass and soot composition enabled us to derive an improved pressure drop correlation. The assumption of constant particulate layer permeability proves unable to predict the influence of the gas temperature on the pressure drop. This discrepancy seems to be linked to the high Knudsen number of the flow in the particulate layer. A new correlation is proposed. This correlation contains four adjustable constants which have been determined on a single experimental run. Without modifying these constants, other cases have been correctly simulated. Obviously, more work is needed to substantiate this approach. In a second step, regenerations with and without additive (Cerium) for two different soot compositions have been simulated and compared with experimental results. Soluble Organic Fraction vaporization has to be taken into account to obtain the right soot mass when regeneration begins. The experimental trend is well captured by numerical simulations.

DYNAMIC FILTRATION MODELING IN FOAM FILTERS FOR DIESEL EXHAUST

Abstract Diesel Particulate Filters (DPFs) are probably the most effective means for trapping the exhaust emitted particulate from diesel engines. Foam type filters become a promising alternative to the common wall flow filters, since they are effective in filtering small size particles and provide a larger specific surface area for catalytic coatings. A mathematical model taking into account the significant phenomena during the dynamic filtration of foam fitters is developed. The model predicts the filtration efficiency and the induced backpressure as function of the geometric filter properties and operating conditions. A novel approach is employed which considers both “deep-bed” and “cake”filtration characteristics in the filter. Due to the particular structure properties of the foam filters it is necessary to define a characteristic parameter, which differentiates among different filter types. This parameter, which has a physical meaning, is easily derived by simple experimental measurements. The model is employed to. identify and understand the critical parameters of the phenomenon. Indicative parametric runs are presented, which illustrate the applicability of the model in system optimization procedures.

Experimental and modeling study on zeolite catalysts for diesel engines

The work presented in this paper was aimed at detecting, understanding and modeling some critical behavior aspects of zeolite-containing diesel catalysts. An already available mathematical model for precious metal catalysts was used as a starting point. A specially designed set of experiments provided the information needed to improve certain modeling features. New submodels were introduced to account for hydrocarbon and H2O adsorption, as well as diffusion limitations in the zeolite. The effect of flow maldistribution during real world operation was investigated experimentally and computationally. Although a number of issues (especially regarding the DeNOx mechanisms) are not yet fully resolved, significant progress was achieved as regards the understanding and computational prediction of diesel catalyst operation.

Computer aided assessment and optimization of catalyst fast light-off techniques

Abstract Strict future legislation standards are forcing the car industry to employ new techniques for reducing exhaust gas emissions. Most of these techniques focus on accelerating the appearance of catalyst light-off and are thus called fast light-off techniques (FLTs). Optimized exhaust systems comprising FLTs will be able to meet the forthcoming legislation standards for the United States [low emissions vehicle (LEV) and ultra-low emissions vehicle (ULEV)] and the European Union (Stage III). The most promising active and passive FLT systems are briefly reviewed. Computer aided optimization of such systems can be realized with the help of specific computational tools, which are briefly presented in this paper, and according to a concept optimization methodology, which is also discussed. The results indicate an increased sensitivity of FLT systems’ performance over the selected values of certain design and operating parameters that were featured in the examples presented. Moreover, by comparing the computational results with knowledge gained from experiments and testing, the paper indicates that, given the suitable computational tools, the optimization procedure can take place in a most cost-effective manner by substituting many experiments with computer test case runs.

Detection of Automotive Catalysts Failure by Use of On-Board Diagnostics

Although major emission reductions from gasoline vehicles have been achieved with the help of catalytic exhaust gas after-treatment, durability of the systems involved is still difficult to guarantee with high confidence levels. On-board diagnosis (OBD) of the main emission control systems is expected to play a central role in any modern emission reduction policy. An overview of theoretical principles regarding catalytic converter operation characteristics, catalyst activity deterioration and OBD system requirements is initially presented. The most important existing approaches to catalytic converter on-board diagnosis are examined by means of presenting principles of operation, assessment methodologies and practical problems associated with each of them. The techniques examined cover dual lambda sensor, hydrocarbon and temperature measurement methods, some of which have already been applied to conform to transitional OBD regulations. It appears, however, that significant work remains to be carried out in order to develop reliable and cost-effective catalytic converter OBD technology to cover legislation requirements, especially for modern low-emitting vehicles.