Grigorios C. Koltsakis

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.

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.

Catalyzed diesel particulate filter modeling

Abstract An increasing environmental concern for diesel particulate emissions has led to the development of efficient and robust diesel particulate filters (DPF). Although the main function of a DPF is to filter solid particles, the beneficial effects of applying catalytic coatings in the filter walls have been recognized. The catalyzed DPF technology is a unique type of chemical reactor in which a multitude of physicochemical processes simultaneously take place, thus complicating the tasks of design and optimization. To this end, modeling has contributed considerably in reducing the development effort by offering a better understanding of the underlying phenomena and reducing the excessive experimental efforts associated with experimental testing. A comprehensive review of the evolution and the most recent developments in DPF modeling, covering phenomena such as transport, fluid mechanics, filtration, catalysis, and thermal stresses, is presented in this article. A thorough presentation on the mathematical model formulation is given based on literature references and the differences between modeling approaches are discussed. Selected examples of model application and validation versus the experimental data are presented.

Intra-layer temperature gradients during regeneration of diesel particulate filters

Wall-flow filters are worldwide recognized as the most efficient devices for the abatement of particulate emissions from automotive diesel engines. Mathematical models simulating the particulate thermal oxidation process in the filters are already applied for system optimization. This paper deals with the appropriateness of a specific assumption inherently used in all relevant published models, namely the temperature uniformity in the soot and wall layer. A new mathematical model is developed to predict the temperature gradients under various operating conditions. Based on the model results, it is shown that significant temperature gradients inside the soot layer may exist under some practical operating conditions. These conditions are associated with high flow rates and high soot loadings. In these cases, the uniform temperature assumption may lead to erroneous results for the prediction of the overall regeneration process. The error of this assumption is assessed as function of the soot porosity.

Diesel particulate filter pressure drop Part 1: modelling and experimental validation

Abstract Pressure drop modelling is a subject of special interest for the design and control of diesel particulate filters. Based on previous experience, an improved pressure drop model is presented. Special emphasis is given on the soot permeability properties and its dependence on temperature and pressure. With the assumption of uniform wall flow distribution throughout the channel length, it is possible to derive an analytic expression for pressure drop calculation. The main difference with previously proposed analytic expressions lies in the inclusion of gas density dependence on local pressure, which necessitates an iterative calculation procedure. The importance of this improvement is illustrated parametrically. The new model is validated against experimental data on an engine bench, using a double filter configuration to ensure constant filter soot loading throughout the test.

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.

Study of Catalytic Regeneration Mechanisms in Diesel Particulate Filters Using Coupled Reaction-Diffusion Modeling

Diesel particulate filters are today widely accepted as a viable technology for drastically reducing particulate emissions from diesel engines. Current applications are based on some form of catalytic assistance for the filter regeneration purposes, either in the form of a fuel borne catalyst or by employing catalyzed filters. This paper presents an experimental and computational study of the prevailing reaction mechanisms in the catalyst supported DPF systems. The knowledge of the soot reaction kinetics in uncatalyzed filters with O 2 and NO 2 is a prerequisite in this respect. Next,\the reaction rates in the case of using a Ce-based fuel-borne catalyst are evaluated. Emphasis is given on the importance of oxygen diffusion effects during uncontrolled regeneration. Finally, the regeneration mechanisms in a catalyst coated filter are studied. In this case, experiments and simulations are carried out for controlled and uncontrolled regenerations, revealing differences in the governing reaction-diffusion processes. The diffusion of NO 2 , which appears important for low temperature regenerations is studied parametrically in more detail. Concluding, each configuration exhibits different characteristics regarding the reaction mechanism and its temperature dependence, with important implications for the system designer of diesel catalytic after-treatment systems.

Warm-up behavior of monolithic reactors under non-reacting conditions

Abstract The transient heat transfer characteristics of monolithic reactors are of major importance because the time for the reactor to reach the required reaction temperatures should be minimized. A common application is the automotive catalytic converter. A simple numerical model for the heat transfer in an adiabatic thin wall channel is presented, taking account of the gas-solid convection, axial wall conduction and heat capacity effects in the solid phase. With the aid of the numerical model and the dimensional analysis, the thermal response of the channel after a step change of the inlet feed gas temperature is studied. For the case of negligible axial conduction, the results can be expressed in a single curve using suitably modified coordinates. The critical design and operating parameters are recognized and discussed. The results can be also expressed using common engineering variables to provide reliable assessment of several design and operating parameters of a monolithic reactor, such as void fraction, cell density, space velocity, etc.

Diesel particulate filter pressure drop Part 2: On-board calculation of soot loading

Abstract In the first part of this study, an improved model for the calculation of pressure drop in particulate filters was presented with special emphasis on the soot permeability properties and its dependence on temperature and pressure. With the assumption of uniform wall flow distribution throughout the channel length, it was also possible to derive an analytical expression that was validated against experimental data on an engine bench. Using the results of pressure drop modelling, it is possible to carry out ‘inverse’ calculations of the soot mass in the filter based on the measurements of flowrate, temperature and backpressure, which forms the main subject of the present work. This approach is tested successfully against experimental data in filters of different geometry and material, yielding promising results. Such computations may be very helpful for the interpretation of experimental results in engine testing. In this paper, typical applications in the field of reaction rate assessment are presented. Moreover, model-based soot loading calculations are expected to assist the implementation of regeneration and on-board diagnostics strategies for particulate filters.