Tariq Shamim

A Comprehensive Model to Predict Three-Way Catalytic Converter Performance

This paper describes the development of a comprehensive mathematical and numerical model for simulating the performance of automotive three-way catalytic converters, which are employed to reduce engine exhaust emissions. The model simulates the emission system behavior by using an exhaust system heat conservation and catalyst chemical kinetic submodel. The resulting governing equations are solved numerically. Good agreements were found between the numerical predictions and experimental measurements under both steady-state and transient conditions. The developed model will be used to facilitate the converter design improvement efforts, which are necessary in order to meet the increasingly stricter emission requirements. @DOI: 10.1115/1.1424295#

The effect of Lewis number on radiative extinction and flamelet modeling

This study investigates the influence of Lewis number on radiative extinction and flamelet modeling. The interaction of Lewis number with different transient effects, such as fluctuating reactant concentrations, fluctuating reactant temperatures, and variable partial premixing, are considered. The results underscore the importance of including the effect of non-unity Lewis numbers and their interaction with chemistry and unsteadiness in improving the predictive capability of flamelet combustion modeling approach, and in precise determination of radiation-induced extinction limits. An increase of Lewis pushes the radiation-induced extinction limit, which occurs at low strain rates, toward higher values of strain rates.

The Effect of Space Velocity on the Dynamic Characteristics of an Automotive Catalytic Converter

This paper presents a computational investigation of the effect of space velocity on the dynamic performance of an automotive catalytic converter. The objective is to develop a better fundamental understanding of the converters performance under actual driving conditions. The study employs a single-channel based, one-dimensional, non-adiabatic model. The transient effects are considered by modulating the air-fuel ratio and compositions of the exhaust gases entering the catalyst. The results elucidate the role of space velocity in determining the catalyst behavior under transient conditions. At high space velocities, the catalyst performance is relatively more influenced by imposed transients.

Comparison of Chemical Kinetic Mechanisms in Simulating the Emission Characteristics of Catalytic Converters

Engine exhaust systems need to undergo continuous modifications to meet increasingly stricter regulations. In the past, much of the design and engineering process to optimize various components of engine and emission systems has involved prototype testing. The complexity of modern systems and the resulting flow dynamics, and thermal and chemical mechanisms have increased the difficulty in assessing and optimizing system operation. Due to overall complexity and increased costs associated with these factors, modeling continues to be pursued as a method of obtaining valuable information supporting the design and development process associated with the exhaust emission system optimization. Insufficient kinetic mechanisms and the lack of adequate kinetics data are major sources of inaccuracies in catalytic converters modeling. This paper presents a numerical study that investigates the performance of different chemical mechanisms in simulating the emission conversion characteristics of catalytic converters during both steady state and transient conditions. The model considers the coupling effect of heat and mass transfer with the catalyst reactions as exhaust gases flow through the catalyst. The heat transfer model includes the heat loss due to conduction and convection. The effect of radiation is assumed to be negligible and is not considered. The resulting governing equations based on the conservation of mass, momentum and energy are solved by a tridiagonal matrix algorithm (TDMA) with a successive line under relaxation method. The performance of different chemical kinetic schemes is reviewed by comparing the results of numerical model with the experimental measurements.

Dynamic Response of Automotive Catalytic Converters to Modulations in Engine Exhaust Compositions

This paper presents a computational investigation of the effect of composition modulations on an automotive catalytic converter. The objective is to develop a better fundamental understanding of the converter’s performance under actual driving conditions. Such an understanding will be beneficial in devising improved emission control methodologies by exploiting the catalyst transient behavior. The study employs a single-channel based, one-dimensional, non-adiabatic model. Two types of imposed transients (sinusoidal and step changes) are considered. The results show that composition modulations cause a significant departure in the catalyst behavior from its steady behavior, and modulations have both favorable and harmful effects on pollutant conversion. The departure is relatively significant for catalyst CO and HC conversion performance. The operating conditions and the modulating gas composition have substantial influence on catalyst behavior. Near stoichiometric condition, modulations of HC concentration have relatively greater effect and result in increased CO and HC conversions. Modulations of CO, on the other hand, result in a decrease of CO conversion. The effect of CO modulations on HC conversion is slightly positive. For the conditions studied, NO modulations generally do not result in any significant change in catalyst performance.Copyright

The effect of time-dependent partial premixing in radiating flamelets

An understanding of the effect of partial premixing on flame characteristics is important to improve the design of many practical applications. In this study, a numerical investigation is carried out to examine the effect of time-dependent partial premixing in counterflow diffusion flames. The objective is to help understand the effect of turbulent fluctuations on a flamelet embedded in the flow field. Premixing of both air with the fuel stream and of fuel with the air stream is considered. The equivalence ratio of the partially premixed reactants is varied as a function of time. As expected, unsteady partial premixing results in a fluctuating double-flame configuration: a premixed flame and a diffusion flame. The maximum flame temperature, heat release rate and radiative heat loss are used to describe the flame response to fluctuations in partial premixing. The increase in partial premixing raises the diffusion flame temperature. This increase in temperature is greater at higher strain rates due to closer proximity of the premixed flame. The flame dynamics to changes in partial premixing is influenced by similar physical processes as those responsible for flame dynamics of pure diffusion flames subjected to unsteady effects. At low fluctuation frequencies, the flame responds quasi-steadily to changes in partial premixing. However, the amplitude of the flame response decreases gradually with an increase in the fluctuation frequency. At large frequencies, the flame is effectively insensitive to changes in partial premixing. A modified Strouhal number (based on the ratio of imposed fluctuation frequency and strain rate) is defined to predict the flame response to imposed fluctuation in partial premixing and to identify a regime where transient effects are important.

Modeling and simulation of automotive catalytic converters

This paper describes the development of a comprehensive mathematical and numerical model for simulating the performance of automotive three-way catalytic converters, which are employed to reduce engine exhaust emissions. The model simulates the emission system behavior by using an exhaust system heat conservation and catalyst chemical kinetic sub-model. The resulting governing equations are solved numerically. Good agreements were found between the numerical predictions and experimental measurements under both steady state and transient conditions. The developed model is employed to investigate the converter dynamic response during transient driving conditions. The transient conditions are simulated by considering modulations in the air-fuel ratio.

Control of Cold Start Emissions through Modulation of Engine Exhaust Gases

This paper presents a computational investigation of the effect of engine exhaust gas modulations on the performance of an automotive catalytic converter during cold starts. The objective is to assess if the modulations can result in faster catalyst light-off conditions and thus reduce cold-start emissions. The study employs a single-channel based, one-dimensional, non-adiabatic model. The modulations are generated by forcing the variations in exhaust gases air-fuel ratio and gas compositions. The results show that the imposed modulations cause a significant departure in the catalyst behavior from its steady behavior, and modulations have both favorable and harmful effects on pollutant conversion during the cold-starts. The operating conditions and the modulating gas composition have substantial influence on catalyst behavior.

The Effect of Engine Exhaust Temperature Modulations on the Performance of Automotive Catalytic Converters

This paper presents a computational investigation of the effect of exhaust temperature modulations on an automotive catalytic converter. The objective is to develop a better fundamental understanding of the converter’s performance under transient driving conditions. Such an understanding will be beneficial in devising improved emission control methodologies. The study employs a single-channel based, one-dimensional, non-adiabatic model. The transient conditions are imposed by varying the exhaust gas temperature sinusoidally. The results show that temperature modulations cause a significant departure in the catalyst behavior from its steady behavior, and modulations have both favorable and harmful effects on pollutant conversion. The operating conditions and the modulating gas composition and flow rates (space velocity) have substantial influence on catalyst behavior.Copyright

Regeneration Characteristics of Diesel Particulate Filters Under Transient Exhaust Conditions

Particulate emissions from diesel engines, which have hazardous effects on living beings and environment, can be controlled by employing diesel particulate filters (DPFs). The DPF cleans the exhaust by physical trapping of the particulates. A major challenge in developing a DPF with wider applications is its lower durability. The filter durability may be increased by careful design of regeneration (soot oxidation) strategies. The regeneration characteristics of a DPF under steady state conditions are well known. However, during a typical driving cycle, a DPF is subjected to highly transient conditions due to changes in driving modes. These transients result in fluctuations of exhaust flow rate, gas composition and temperature. Such modulating exhaust conditions make the DPF performance and regeneration characteristics differ significantly from that under steady state conditions. The objective of this paper is to investigate the thermal and catalytic regeneration characteristics of DPF under transient exhaust conditions. In this work, a computational investigation is conducted to determine the effect of temperature and exhaust flow modulations on a DPF. The paper contributes to a better fundamental understanding of the filter’s performance under transient driving conditions.Copyright