A. M. Stamatelos

Catalytic automotive exhaust aftertreatment

Abstract Catalytic exhaust aftertreatment of vehicle engines is increasingly employed to the benefit of the atmosphere quality, especially in the large urban area of the world. Both spark-ignition and compression-ignition engines benefit from the application of catalytic converters for the elimination of their main pollutants. Catalysts are further employed in various forms as regeneration aids in particulate filters of diesel engines. The especially demanding exhaust gas conditions prevailing in each engine application pose challenging problems to the emissions control engineer. The attainment of strict emissions regulations requires highly active and durable catalysts, as well as optimized exhaust system design and engine controls. This paper reviews the potential of catalytic systems in automobile emission control. The review covers the catalyst technology applicable in each case, the operating principles and performance characteristics, durability aspects and considerations regarding the interactions between catalyst performance and engine management. The concise presentation of related mathematical model equations provides insight into the catalytic mechanisms and the physical phenomena involved. Further reductions of catalytically controlled automobile emissions may be attained by developing improved and more durable catalysts, by applying a systems approach in designing optimized engine-exhaust aftertreatment configurations, as well as by efficient control of in-use catalytic systems through inspection, maintenance and on-board diagnostics.

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.

Thermogravimetric analysis of soot emitted by a modern diesel engine run on catalyst-doped fuel

Understanding the mechanisms that affect catalytic activity in porous ceramic diesel particulate filters (DPF) at the temperature range 200 to 400°C is important for the successful modeling of the initiation and evolution of catalytic regeneration by use of fuel additives. This refers not only to the dry carbon particulate, but also to the volatile hydrocarbons adsorbed on it. In this paper, a detailed analysis of the hydrocarbon adsorption-desorption and oxidation behavior of diesel particulate emitted by a modern diesel engine and collected on a SiC diesel filter is performed by use of thermogravimetric and differential scanning calorimetry analysis (TGA-DSC). Non-isothermal tests were performed with samples collected directly from a ceramic filter connected to the exhaust system of the diesel engine running under low and medium speed and load operating conditions with and without fuel additive. Fuel additive concentration was varied to investigate its effect on the soot oxidation behavior. Based on the TGA data, the kinetic parameters of the soot oxidation reaction were calculated. The effect of volatile adsorbed hydrocarbons on the soot oxidation reaction was evaluated by comparing the calculated activation energies for samples collected from the center and the periphery of the filter at various exhaust temperatures prevailing at filter loading phase. In particular it was seen that the catalytic activity of the fuel additive is enhanced by the presence of the volatile organic components.

A review of the effect of particulate traps on the efficiency of vehicle diesel engines

Particulate traps are becoming more widely used on city buses, some delivery trucks and fork lift trucks. The possible use of diesel particulate traps will lead to a fuel consumption penalty imposed on the baseline engine that is due to the trap back pressure as well as to the energy requirements of the regeneration technique adopted to incinerate the collected soot at will. The combined effect of trap back pressure imposed on the engine and additional energy required for trap regeneration on the overall efficiency of the diesel power plant is examined in this paper. This effect varies according to engine type, trap type and size, regeneration system used, and the vehicle driving mode. Because of the strong interaction among the above parameters, optimization of trap systems on efficiency grounds is complicated. This complexity is even more pronounced in the case of diesel-powered passenger cars, where the full exploitation of their efficiency advantage over gasoline-powered cars is constrained by the necessity of an optimized solution of the particulate emission problem. The main diesel particulate trap regeneration philosophies existing today are reviewed in terms of their effect on the total efficiency of the diesel power plant. This is done by means of representative examples, concerning systems which may be suitable for large-scale application. The conclusions indicate that the price that must be paid for environmental protection, in the case of diesel particulate control systems, may be substantially reduced by system design optimization.

Engine exhaust system design based on heat transfer computation

The heat transfer conditions in automotive exhaust piping are only recently being studied in depth because of their important role in the design and optimization phases of exhaust after-treatment systems. 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. Experimental data from steady state and transient heat transfer measurements in automotive exhaust systems are presented and analyzed by means of a comprehensive transient computer model covering all exhaust piping configurations (single wall, double wall with air gap or insulation) already presented elsewhere. 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 is already integrated in a CAE package for the support of exhaust system design optimization.

Effect of climatic conditions on the design optimization of heat pump systems for space heating and cooling

Domestic heating and cooling is responsible for a fair percentage of world energy consumption. Heat pumps offer the most energy efficient way to provide heating and cooling in many applications, as they can use renewable heat sources of the buildings surroundings. A number of related heat pump technology versions exist that have led to significantly lower heating and cooling energy consumption in certain climatic conditions. In this paper, a comparative discussion is given of the effect of climatic conditions on applying ground source heat pump technologies. Specific examples are given for northern and southern parts of Europe. It is shown that the attainable building energy consumption reduction with ground source heat pump systems may be significantly higher in the warmer Mediterranean climatic conditions. To this end, advanced technology residential heat pump systems should be employed and their operation matched to the specific climatic conditions. It is concluded that climatic conditions significantly affect the performance of heat pump systems, which should lead to markedly different strategies for domestic heating and cooling, if an optimization is sought on sustainability grounds.

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.

Experimental investigation of the pressure drop in porous ceramic diesel particulate filters

Abstract Understanding of the mechanisms that affect flow and pressure drop in porous ceramic diesel particulate filters is important in the design optimization of this class of diesel exhaust after- treatment systems. Furthermore, determination of the parameters involved in the calculation of pressure drop as a function of collected soot mass is important for successful filter loading and regeneration modelling. This paper presents the results of an experimental analysis of pressure drop as a function of the geometric and operating parameters of cordierite and SiC diesel filters. Single- cell filters from cordierite and silicon carbide were prepared to single out any effects from the complex flow processes that take place in a full-sized filter. The product of soot layer permeability and density was experimentally determined by employing a specially designed experimental apparatus. The calculation was supported by a simple computer calculation that is also presented in this paper. The distribution of soot loading inside the channels of a full-sized filter, in various loaded and partially regenerated conditions, was assessed by connecting the apparatus to discharge through selected channels of the filter. The results are shown to improve understanding of the effects of partial regeneration and fuel additive residuals on filter back pressure and flow and soot loading distribution.

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.