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- 7.1: Catalytic Converters
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Today restrictions on pollutant emissions require the use of catalyst-based after-treatment systems as a standard both in SI and in Diesel engines. The application of monolith cores with a honeycomb structure is an established practice: however, to overcome drawbacks such as weak mass transfer from the bulk flow to the catalytic walls as well as poor flow homogenization, the use of ceramic foams has been recently investigated as an alternative showing better conversion efficiencies even accepting higher flow through losses.
The scope of this paper is to analyse the effects of foam substrates characteristics on engine performance. Engine developed by the authors has been enhanced improving the heat exchange model of the exhaust manifold to take account of thermal transients and adding an original 0D model of the catalytic converter to describe mass flows and thermal processes.
The model has been used to simulate a 1. Effects of honeycomb and foam substrates on fuel consumption and on variations of catalyst temperatures and pressures are compared in the paper. As a matter of fact, in order to allow these technologies to be really effective, a proper and concurrent design of plant layout, control systems and management strategies is needed. The complexity of systems and the large number of control variables require a deep understanding of processes that determine the behavior of the controlled powertrain as a system as a whole.
The design of system architecture and of its control devices definitely need a solid theoretical support from physical models to outline system overall behavior, which is mostly non-linear and therefore difficult to predict.
Mathematical models are powerful tools to estimate the influence of system layout and control strategies on the final result thus shortening the way from design specifications to on-road testing Guzzella and Onder, The application of fast mathematical models in the design of powertrains and related management systems is well-known for more than a decade and several examples can be found in the literature Gambarotta and Lucchetti, A comprehensive scenario is outlined in Guzzella and Onder, Most commercial tools are based on these methodologies as reviewed in Gambarotta and Lucchetti, , This scenario highlights the significant role of fast mathematical models in the simulation of complex systems, whose overall behavior arises from the interactions of different components and processes in a complex and not trivial way.
Particular attention has been given to foams as an innovative material for substrates Bach and Dimopoulos Eggenschwiler, Obtained results are shown in the paper.
Open cell foams are cellular materials composed by interconnected solid struts arranged in cells that enclose void regions, and open window or pores. Such foams can be readily manufactured with different technologies and materials ranging from polymers, ceramics Al 2 O 3 , cordierite, or SiC and metals Santoliquido et al. Open cell foams are innovative substrates characterized by high porosity, low density and high mechanical strength.
In recent years they have been considered for various industrial applications like filters, thermal insulators, mechanical energy absorbers, silencers, heat exchangers, and catalytic reactors.
As catalyst substrates they present several advantages over honeycomb monoliths and packed beds. The open cell structure allows higher flow uniformity, which is a critical factor for the pollutant conversion efficiency and for the catalyst durability Zygourakis, ; Martin et al.
In honeycomb monoliths the laminar flow in channels results in low heat and mass transfer. Instead, the network of solid struts of the open cell lattices is characterized by tortuous paths that enhance gas-wall interactions and contribute to lower thermal inertia Giani et al.
In automotive applications a critical parameter is the pressure drop, which affects engine efficiency. Foams have higher pressure drop compared to a monolith with the same dimensions Twigg and Richardson, ; Lucci et al.
This can be compensated by an increased mass transfer that allows downsizing the catalyst Dimopoulos Eggenschwiler et al. Some effort has been spent on their modeling. On the one side, high fidelity CT Computerized Tomography foam scans have been analyzed, on the other side, in order to reduce the computational load, foams have been modeled as regular structures with Kelvin cells Boomsman et al.
It has been demonstrated that regular Kelvin cell substrates perform better than their corresponding randomized foams in terms of trade off between mass transfer and pressure drop Lucci et al. Recently, a variation of foam structure has been proposed based on the advances of additive manufacturing AM techniques.
Different unit cells have been proposed building interconnected structures. Papetti et al. It is not straightforward to quantify the influence of catalyst substrate structure on engine performance due to the different dynamic behavior of honeycombs and foams during transients and to the high non-linearity of the overall engine system.
To compare the influence of honeycomb and foam substrates an original 0D mathematical tool has been developed and used to model an up to date 1. Simulation results obtained with reference to an EUDC driving cycle are reported in the paper showing the effects of these different supports on catalyst thermal transients and on fuel consumption.
For the purpose of this work the engine model described in Gambarotta et al. Combustion is considered defining a proper Heat Release Rate HRR and pollutant formation is estimated through black-box sub-models. Fuel system model takes account of the fuel rail dynamics through its bulk modulus , of injectors flow characteristics and of leakages and allows to calculate injected fuel flow rate from rail pressure p rail and Energizing Time ET.
Black-box map-based models have been used for compressor C and variable geometry turbine VGT. The model and its causality scheme are described in Gambarotta et al. It has been used for the simulation of several automotive engines both SI and Diesel calibrated and validated comparing model output with experimental data, as reported in detail in Gambarotta and Lucchetti , and Gambarotta The proposed model has been also used in an original PC-based Hardware-in-the-Loop HiL system developed by the authors Gambarotta et al.
Heat transfer processes in the exhaust system have a key role in the simulation of ICEs due to the significant influence of exhaust gas temperature on after-treatment systems efficiency. Therefore, a careful description of heat exchange processes is fundamental especially during critical transients e.
Other emission critical phases of engine operation are long time operation at low load, when the after-treatment system is significantly cooled down, as well as at highest load, when temperatures are high enough but exhaust mass flow rates force the catalyst to operate under mass transfer deficiency. For this reason, although within the limitations imposed by a 0D approach, particular attention has been dedicated to the simulation of thermal behavior of the exhaust system.
Working fluid has been considered as a mixture of perfect gases defined through a vector of mass concentrations X mi referred to 7 chemical species, i.
Pr is estimated with the following expression Heywood, :. Temperature and pressure are obtained from the equations of conservation of mass and energy applied to the manifold considered as a 0D volume. Estimating heat flow through manifold walls as suggested in Guzzella and Onder , energy conservation equation for exhaust gases inside the manifold can be written as follows:.
Enthalpy of gases leaving the manifold h tur and h EGR are calculated assuming that gas temperatures are equal to that inside the manifold. In the presented model the thermal inertia of the exhaust manifold has been considered assuming a defined overall mass m w and a constant specific heat c w for the manifold walls Figure 1.
Manifold walls temperature has been assumed uniform, and its changes have bene estimated through the following differential equation:. These heat fluxes can be calculated with reference to the well-known schematic description reported in Figure 1 , where heat is exchanged by convection and radiation between gas flow and internal walls, by conduction through the walls and by convection and radiation between external walls and ambient air.
In the proposed model, however, internal radiation has been considered negligible. Even if the real geometry of the manifold is complex, it has been modeled as a single cylindrical pipe with a proper length L to keep the calculation burden within the limits of the 0D approach. The term Pr c often assumes a value close to 1 and values for a and b are defined from measurements.
The value of Nu was estimated from the Gnielinski correlation reported in Konstantinidis et al. The last value can be estimated through well-known correlations from Konstantinidis et al. The estimation of convective heat flux from manifold walls to ambient air is more difficult due to the component geometry and to external flow pattern.
For the sake of simplicity, manifold geometry has been assumed as cylindrical and external flow field uniform and related to the vehicle speed.
The model is based on the correlation proposed in Konstantinidis et al. From Nu out convection coefficient and heat flux can be calculated since. Therefore, it can be calculated through the well-known Stefan-Boltzmann relationships Incropera et al. A catalytic converter is a complex component from the point of view of both gas flow pattern and of chemical reactions. Fluid dynamics, heat and mass transfer processes have a significant role in its behavior and should be carefully considered.
Taking account of the aims of the presented work, neither a 3D e. A 0D approach has been followed assuming for each component a uniform spatial distribution of thermodynamic parameters and applying conservation equations with empirical correlations where required. The developed model proved to be able to simulate catalyst behavior and its influence on powertrain performance during significant transients e. The model has been developed according to the causality reported in Figure 2.
The model of the core in orange was based on a QSF procedure i. Since processes in a catalytic converter are complex and typically three-dimensional, proper assumptions had to be introduced to capture their overall effects still limiting the simulation burden.
At each time step mass flow and temperature changes through the core were estimated solving the two set of algebraic equations from the two modules, which are coupled through heat exchanges between exhaust gas and substrate walls according to Figure 3.
Assuming the catalyst core as a concentrated flow resistance without mass accumulation gas mass flow rate can be estimated through an empirical algebraic correlation in the following form:.
Then gas temperature at the core exit can be defined integrating the energy conservation equation in 1D and in steady state:. Gas properties are evaluated at p m and T m and assumed constant. Figure 4. The convective heat exchange between gas and core is described as usual through a convection coefficient h obtained from Nu estimated with an empirical correlation in the following form Konstantinidis et al.
Wall temperature of the monolith T mon is assumed constant in the time step, i. Molecular diffusion of different species and chemical reactions in the gas mixture and in the core has not been considered. The number N and the type of involved pollutants depend on the specific application.
In the presented model CO and one or more species representative of HC have been considered, since their oxidation reactions were assumed as the most significant in the determination of the catalyst temperature. It should be noted that working fluid has been considered as a mixture of 7 chemical species, i. This approach which is mainly black-box, as usually required by Real-time models allows considering further reactions that may occur in the catalyst by introducing proper empirical correlations for the simulation of different catalytic converters and after-treatment systems.
Therefore, if temperatures are high enough, chemical species can be supposed to react instantly as soon as they reach substrate walls. Assuming a concentration of chemical species in flowing gases that falls exponentially along the axial abscissa, and recalling that diffusive mass exchange is proportional to the difference in concentration, an exponential distribution of heat generated from unburnt compounds has been assumed, expressed in the following form:.
Therefore, the following expressions for a and b are obtained:. Heat flux between gas and monolith in each time step can be estimated through the equation:. It should be noted that since properties of gas mixture are determined with reference to the average temperature in the core, the value of T out is estimated through an iterative calculation do-while procedure, Figure 4 with a 0.
For the estimation of changes of mean temperature of the monolith T mon the energy conservation equation can be used in the following form:. Even if different configurations can be found, the most common technique is to fit the monolith into a metal casing with a layer of interposed insulating material: this layout has been assumed in the developed model, as schematically shown in Figure 5. Heat transfer occurs from the monolith to the ambient air first by conduction through the layer of insulating material and metal casing and then by convection and radiation from the external walls to ambient air.
In this case convection can be either forced or natural depending on the vehicle speed v , which therefore represents an input parameter for the model. Following a Quasi-Steady approach, the steady-state heat transfer process can be simulated within each time step assuming two thermal resistances in series, and the overall thermal resistance can therefore be expressed as:.
Taking into account only the insulating material layer i. Forced convection to ambient air can be considered assuming a cylindrical casing with radius equal to r ext : therefore. The convection coefficient h is obtained starting from Nu Incropera et al.
Then h can be calculated with the following relation:.
Exhaust Systems – Uncorking Your Engine’s Potential | Bolt-On Basics
Depending on the engine platform and design of the manifold, the shape and size can vary. High-quality aftermarket exhausts will add power, increase style and refine the exhaust tone. Use this decibel chart for reference when deciding if an exhaust system is too loud. Most laws determine exhausts to be unlawfully noisy beyond 95dB. The Nissan R35 GT-R had to go through a wind tunnel during testing to ensure there is nearly no resistance It not only delivers exceptional performance, it also sports an unmistakable look.
Scientific Design of Exhaust and Intake Systems (Engineering and Performance) [Philip Hubert Smith, Phillip H. Smith, John C. Morrison] on bhepallianceinc.org
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Is exhaust drone rattling the fillings out of your teeth? Flowmaster's line of mufflers go from the loud and rowdy to exhaust that still sounds right, but is livable on a day-to-day basis. Here's what you need to know in order to select the right case for your exhaust! Read More.
Today restrictions on pollutant emissions require the use of catalyst-based after-treatment systems as a standard both in SI and in Diesel engines. The application of monolith cores with a honeycomb structure is an established practice: however, to overcome drawbacks such as weak mass transfer from the bulk flow to the catalytic walls as well as poor flow homogenization, the use of ceramic foams has been recently investigated as an alternative showing better conversion efficiencies even accepting higher flow through losses. The scope of this paper is to analyse the effects of foam substrates characteristics on engine performance. Engine developed by the authors has been enhanced improving the heat exchange model of the exhaust manifold to take account of thermal transients and adding an original 0D model of the catalytic converter to describe mass flows and thermal processes. The model has been used to simulate a 1.
7.1: Catalytic Converters
A catalytic converter is a device used to reduce the emissions from an internal combustion engine used in most modern day automobiles and vehicles. Not enough oxygen is available to oxidize the carbon fuel in these engines completely into carbon dioxide and water; thus toxic by-products are produced. Catalytic converters are used in exhaust systems to provide a site for the oxidation and reduction of toxic by-products like nitrogen oxides, carbon monoxide, and hydrocarbons of fuel into less hazardous substances such as carbon dioxide, water vapor, and nitrogen gas. Catalytic converters were first widely introduced in American production cars in due to EPA regulations on toxic emissions reductions. Without catalytic converters, vehicles release hydrocarbons, carbon monoxide, and nitrogen oxide. These gases are the largest source of ground level ozone, which causes smog and is harmful to plant life.
Smith,John C. Search and Browse : Transportation : Booksamillion. Smith and John C. John M. John C. Morrison is one of the foremost authorities on the analysis of the induction and exhaust processes of high-speed engines. Together with Philip Smith, he gives a thorough For years, engineers, engine designers, high-performance tuners and racers have depended on the Scientific Design of Exhaust and Intake Systems to develop maximum potential from their engines.
Present day engines are required to have more engine power and are also required to meet the strict pollution standards. In an automobile the exhaust muffler plays an integral role in reducing the sound of the automobile, as well as the ride itself. In order to maintain a desired noise and comfortable ride, the modes of a muffler need to be analysed. Here dynamic modal analyses were carried out to determine the mode shapes, stresses and deformations of exhaust muffler using CAE analysis. Published by American Institute of Science.
A catalytic converter is an exhaust emission control device that reduces toxic gases and pollutants in exhaust gas from an internal combustion engine into less-toxic pollutants by catalyzing a redox reaction an oxidation and a reduction reaction. Catalytic converters are usually used with internal combustion engines fueled by either gasoline or diesel —including lean-burn engines as well as kerosene heaters and stoves. The first widespread introduction of catalytic converters was in the United States automobile market. To comply with the U.
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