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Proceedings of the 37 National & 4th International Conference on Fluid Mechanics and Fluid Power

December 16-18, 2010, IIT Madras, Chennai, India

FMFP2010:581

PERFORMANCE STUDIES OF CATALYTIC CONVERTER USED IN

AUTOMOBILE EXHAUST SYSTEM

Bharath M.S Baljit SinghM.S. Ramaiah School of Advanced Studies, Faculty of Mechanical Engineering,

Bangalore, India, Universiti Teknologi MARA (UiTM),

40450 Shah Alam, Malaysia

bs_dhillon@hotmail.com

P.A.Aswatha NarayanaFaculty of Mechanical Engineering,

Universiti Teknologi MARA (UiTM),

40450 Shah Alam, Malaysia

paa_iitm@yahoo.co

ABSTRACTAn improvement of catalytic converter design requires better fundamental understanding of complex

processes taking place involving fluid flow, heat and mass transfer, and chemical reactions. The paper deals

with the study of fluid flow inside the catalytic converter and the study of temperature distribution andchemical reaction in catalytic converter. CATIAV5R15 was used for geometric modeling of catalytic

converter. Domain discretization was carried out in Gambit 2.2. Fluent 6.2 was used for carrying out analysis.

Flow field in the catalytic converter is influenced by the flow resistance of the substrate for a given geometric

configuration. As the mass flow rate increases, the pressure drop also increases. At lower temperature, thecatalytic converter will be inactive. The heat release due to chemical reaction at lower temperature does not

play a significant role.

Keywords: Catalytic Converter, FLUENT, CFD, Reaction.

INTRODUCTIONAt the turn of the 19th century the door

towards individual mobility was opened with the

start of the mass production of automobiles.During the 20th century, the number of gasoline-

driven cars increased from a few thousand to

several hundred million. This dramatic increase,

which is expected to continue at least for the nextthree decades, is accompanied by a corresponding

growth of pollutants in the atmosphere, since an

internal combustion engine drives almost everyvehicle. These pollutants affect both the

environment and the human health in manyunpleasant ways. Hence it is clear that measures

have to be taken to reduce the levels of the

emissions to tolerable limits. Meeting theincreasingly stringent emission requirements is a

very important challenge faced by the automobile

industry. This challenge makes emission control amajor thrust area in engine research. Emissions

from engines are major sources of urban air

pollution. The engine exhaust gases contain oxides

of nitrogen (NOx), carbon monoxide (CO), andpartially burnt or unburned hydrocarbons (HC).

Proceedings of the 37th National & 4th International Conference on Fluid Mechanics and Fluid Power

December 16-18, 2010, IIT Madras, Chennai, India.

FMFP10 - NE - 21

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These pollutants can be removed from the exhaustgases by employing catalytic converter. Catalytic

converters have been used in automobiles for

several years and various types of them are

available. Their designs, however, need to beimproved to obtain better conversion efficiency in

order to meet the new emission requirements. Soon

it became apparent that the improvement of thecombustion in gasoline engines alone was not

sufficient to reach the desired emission levels.

Therefore, exhaust gas after treatment systemswere introduced, which are capable of completing

the combustion by means of catalysts, thus

reducing the levels of undesired emission

components to very low concentrations. Withgrowing concerns about environmental impact,

legislated requirements for automobile emissions

are becoming ever more stringent. Catalytic

converter is one of the most widely usedcomponents for emission removal from engine

exhaust.The performance of the catalytic converter

is substantially affected by the flow distribution

within the substrate. A uniform flow distribution

increases the efficiency, causes less pressure dropand increases engine performance. Flowdistribution in converter assembly is controlled by

the geometry configurations of inlet and outletcones, the substrate and exhaust gas compositions.

Hence better design of the catalytic converter isnecessary.

Many researchers have studied converter

thermal and conversion characteristics. In most of

previous research only one substrate channel was

modeled and importance was given only to thermaland fluid flow. The reaction on the surface of

substrate was given much importance. With the

advance of high performance computers andaccurate numerical schemes, computational fluid

dynamics (CFD) can be used to simulate the

complex flow physics inside the converter. It canprovide complete, fast and accurate analysis of the

thermal fluid flow cut testing costs and reduces

design circles. In converter designs, CFD is used to

analyse flow distribution, pressure drop, andtemperature profile and chemical reactions.

LITERATURE REVIEWFrancisco et al. [1] studied one-dimensional

fluid dynamic Model for catalytic converter in

automotive engines. The main aim of this paper

was to present a simple approach to the one-dimensional modelling of the fluid dynamic

behaviour of the catalytic converter. They

developed a geometric model that was capable ofcompletely representing the dynamic behaviour of

the converter, that is, its reflection and

transmission characteristics.

Cathy Chung et al. [2] studied the CFDinvestigation of thermal fluid flow and conversion

characteristics of the catalytic converter. Their

main objective was to predict the maximum

operating temperature for appropriate materialsand to develop a numerical model, which can be

adjusted to reflect changes in the catalyst/washcoatformulation to accurately predict effects of flow,

temperature and light off behaviour. They

concluded that by changing the concentrations, the

converter characteristics and steady statetemperature could be changed.

Olaf Deutschmann and Ju.Rgen Warnatz

[3] studied detailed surface reaction mechanism ina three-way catalytic converter. In this paper, two-

dimensional flow field description, includingdetailed reaction mechanism for the conversion ofCO, C3H6, and NO has been used to simulate theexhaust gas treatment in a platinum/rhodium-

coated single channel of a typical three-way

catalytic converter. The simulation is based on theCFD code FLUENT and the chemistry module

DETCHEM, which were coupled for the

simulations performed.Joachim et al. [4] studied three-dimensional

simulation of the transient behavior of three-way

catalytic converter. The main aim was to predictthe exhaust gas emissions as function of time for

varying inlet conditions. Their simulation included

the calculation of the transient three-dimensional

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temperature field of the monolithic solid structureof the converter. For the numerical simulation of

transient behavior of three-way catalytic converter

a newly developed CFD code DETCHEM was

used. For this study, experiments were carried outon an engine test bench. A 4-cylinder 1.6 Litres SI

engine was used.

Windmann et al. [5] studied the impact ofthe inlet flow distribution on the light off behavior

of a 3-way catalytic converter. This paper presents

a three-dimensional transient numerical study ofthe influence of the velocity distribution in front of

the inlet face of the monolith during light off of a

three-way catalytic converter. The difference in the

thermal and chemical behavior due to the shape ofthe velocity distribution is discussed in this paper.

G.Gaiser [6] made an important

contribution by studying the new concept for the

achievement of homogeneous flow distribution incatalytic converter. The homogeneity of flow

distribution in catalytic converter is a majorparameter for the conversion efficiency; the light

off as well as the catalyst ageing is strongly

affected by the local flow distribution. Here the

authors have introduced a new concept forachievement of homogeneous flow distribution incatalytic converter.

Ming Chen et al. [7] presented CFDmodeling of three way catalytic converter with

detailed catalytic surface reaction mechanism. Thispaper presents a 3-D CFD modeling of flow andheterogeneous reactions in catalytic converters.

The pressure and velocity fields in the catalytic

converters are calculated by the state of the art

modeling technique for the flow resistance ofcatalyst substrate. A surface reaction model is

applied to predict the performance of a three-way

Pt/Rh catalyst.It is clear from the literature review that

there is a need to study the performance of

catalytic converter to improve the design of thecatalytic converter. The design of the catalytic

converter has to be improved to obtain better

conversion efficiency in order to meet the newemission requirements.

The specific objectives of this work are to

study the behavior of fluid inside the catalytic

converter and surface reaction with temperaturedistribution in catalytic converter at different

exhaust gas temperatures.

ANALYSISThe catalytic converter model construction

was carried out by reverse engineering technique.Ford IKON car was taken for the study. Based on

the dimensions obtained from reverse engineering

technique CAD modeling was done. The

generation of catalytic converter geometryinvolved wire frame, surface and solid modeling.

The Specification of catalytic converter along with

the porous media is shown below in fig.1. The

porosity value applied was 0.689Geometric modeling was carried out using

CATIAV5R15 and fluid domain was extractedusing PRO-E 2001. The features involved in the

geometric modeling were wire-frame, surface and

solid modeling. The CAD modeling of catalytic

converter is shown in fig. 2.After the extraction of fluid domain,

catalytic converter was imported to Gambit to

generate the grid. The model was discritized alongwith boundary layer. Discretization is the method

of approximating the differential equations by asystem of algebraic equations for the variables atsome set of discrete locations in space and time.

The discrete locations are referred to as the grid or

the mesh. Figure 3 shows the discretization of

geometric model. The number of hexahedralelements was around 23,000.

Specifications are defined with three basic

parameter name, type and entity as in FLUENT.In the present analysis, after the grid generation

boundary condition was applied. FLUENT 5/6 was

used as solver. Mass flow was imposed at the inlet,and pressure applied at the outlet. Table 1 shows

the boundary specifications of a catalytic

converter.

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A porous media model was used to treat thesubstrate in the flow analysis. This porous media

simulates pressure resistance to the fluid. The flow

with in the substrate channels is assumed to be a

fully laminar flow [7]. The parameters used in thisporous media were calculated based on the cell

density, and wall thickness. The standard k-

model was selected to account for the turbulentflow. The Reynolds number calculated indicated

that the flow is turbulent. Turbulent flows are

characterized by fluctuating velocity fields. Thesefluctuations mix transported quantities such as

momentum, energy, and species concentration, and

cause the transported quantities to fluctuate as

well. The turbulence kinetic energy, k, and its rateRI GLVVLSDWLRQ DUH REWDLQHG IURP WKH Iollowing

transport equations [10].

-(1)

-(2)

The turbulent (or eddy) viscosity, t, is computed

E\FRPELQLQJNDQGDVIROORZV

------(3)

The porous media model incorporates anempirically determined flow resistance in a region

of model defined as porous. In essence, the

porous media model is nothing more than an addedmomentum sink in the governing momentum

equations. As such, the following modelling

assumptions and limitations should be readilyrecognized [10]. Since the volume blockage that is

physically present is not represented in the model,

Fluent uses a superficial velocity inside the porousmedium, based on the volumetric flow rate, to

ensure continuity of the velocity vectors across theporous medium interface. The effect of the porous

medium on the turbulence field is onlyapproximated. Porous media are modelled by the

addition of a momentum source term to the

standard fluid flow equations. The source term iscomposed of two parts: a viscous loss term, and an

inertial loss term. The momentum equation for the

Darcy law is shown below.

------(4)where, Si = source term for momentum equation

= viscosity of air

YLVFRXVORVVLQWRSRURXVPHGLDv = velocity of the air

C2 = Internal loss factor

'HQVLW\RIDLU

QWKLFNQHVVRISRURXVPHGLDFluent also allows the source term to be modeled

as a power law of the velocity magnitude:

---(5)

where, C0 and C1 are user defined empirical

coefficients.Fluent uses a control-volume-based

technique to convert the governing equations to

algebraic equations that can be solved numericallyfor fluid velocities, mass flow, pressure,

temperature and turbulence parameters and fluid

properties. This control volume technique consists

of integrating the governing equations about eachcontrol volume, yielding discrete equations that

conserve each quantity on a control-volume basis.The discretization scheme of the continuity and

momentum equations and their solutions can be

obtained using segregated solver. Implicit

formulation is used so that the equations are solvedsimultaneously to give the unknown quantities.

The oxidation reaction of CO and hydrocarbons

and reduction reaction of NO were considered. Thehydrocarbons were represented by propylene,

which is easily oxidized hydrocarbon, constituteabout 80% of the total hydrocarbons found in thetypical exhaust gas [2]. The chemical reactions

modeled are shown below, this kind of global

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chemistry has been used in the studies described inthe literature [2].

For oxidation:

2CO+O2 &22 -------(6)

2C3H6+9O2 &22+6H2O -------(7)

For Reduction:CO+NO &22 + 1/2N2 -------(8)

RESULTS AND DISCUSSIONSThe behavior of the fluid inside the

catalytic converter is discussed here. The inlet and

outlet cone of catalytic converter are straight,

hence a good flow is expected. It is found from thecomputation that the flow is very uniform in the

substrate. The maximum pressure drop occurs due

to porous media. The total pressure drop of

catalytic converter is about 7kPa, of which 88% isfrom the substrate, 7% from the inlet cone, and 5%

from the outlet cone.The flow in the catalytic converter is

determined by the geometrical configuration, the

flow resistance characteristics of the substrate and

the Reynolds number [9]. From the pressurecontours, as shown in fig. 4, it is clear that whenthe mass flow rate increases, the pressure drop also

increases. This is due to the presence of porousmedia. From the contours, it is observed that the

pressure near the inlet of porous media is morecompared to the outlet of porous media. Thus theporous media greatly influences the pressure drop.

Pressure contours indicates that the calculated flow

uniformity index at the front face of the substrate is

decreased with the increasing of the Reynoldsnumber, and increased with the increasing of the

cell density. The higher pressure is located around

the catalyst entrance and the centerline.From the velocity contours in fig. 5, it is

seen that there is reverse flow occurring at the inlet

cone of catalytic converter, i.e. there is a largerecirculation zone is formed at the

inlet cone. When the flow enters the porous zone,

it aligns with the channel direction. From figure 6,

it is clear that as the mass flow increases the

pressure drop also increases.The pressure drop obtained from the

computational values was nearly double the values

of pressure drop of the experimental values. This isbecause the experimental values were of monolith

catalytic converter, i.e. it has single substrate

whereas in the present analysis had the three waycatalytic converter, and it has two substrates.

Hence the computational pressure drop values

were nearly double the experimental pressure dropvalues [7]. The experimental v/s computational

curve is shown in fig.7.

The incoming hot exhaust gas heats up the

monolithic structure. At the early stage ofoperation, the heat is primarily provided by the

heat capacity of the incoming exhaust gas. Heat

release due to chemical reactions does not play a

significant role, which is also revealed by theconversion of C3H6 as shown in fig. 8. After the

converter has reached its operating temperature,the exit gas temperature exceeds the incoming gas

temperature caused by exothermic reactions. When

the temperature of the exhaust gas is 500K, not

much conversion takes place. Temperature at theinlet is more than the outlet. When the temperaturestarts increasing, the temperature at the outlet also

increases. From fig. 8, it is clear that thetemperature at the outlet is increasing, once the

catalytic converter exceeds 700K the exit gastemperature will be more than inlet temperature.

Here, chemical heat release leads to thetemperature increase as the exhaust gas flows

through the substrate channels.

The computed conversion efficiencies ofCO, and NO as a function of inlet gas temperature

is shown in the Table 2. From the table, it is clear

that the conversion of CO, and NO begins at 400K.At 800K reach the maximum value. The

conversion efficiency of C3H6 is about 88% when

the temperature is at 800k.The conversion of COstarts at 400K and increases up to 44% at 800K.

The NO conversion is 48% at 800K .The

conversion efficiency is related to the substrate

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temperature and composition of the inlet. At 400K,when the catalyst is still cold, almost no chemical

reaction takes place, which is because at this

temperature, the catalysts will be inactive.

CONCLUSIONThis study investigated the flow characteristics,

the temperature distribution and conversionefficiency of catalytic converter. The

computational tool FLUENT was used to study the

behavior of fluid flow and the conversion rates ofemissions as a function of exhaust gas temperature.

The analysis shows that the flow field in the

catalytic converter is influenced by the flow

resistance of the substrate for a given geometricconfiguration. As the mass flow rate increases the

pressure drop also increases. The conversion

efficiency depends upon the substrate temperature

and composition of the inlet. By increasing thetemperature the conversion efficiency also

increases. At lower temperature the catalyticconverter will be inactive. The heat release due to

chemical reaction does not play a significant role.

ACKNOWLEDGEMENTThe authors wish to thank the reviewers for theircomments.

REFERENCES

[1] Francisco Payri, Jesus Benajes, and JoseGalindo (1999), One-dimensional Fluid DynamicModel for catalytic converter in automotive

engines, SAE-1999-01-0144.

[2] Cathy Chung, Sivanandi Rajadurai and LarryGEE(1999), CFD Investigation of Thermal fluid

flow and conversion characteristics of the catalytic

converter, SAE 1999-01-0462.

[3] Daniel Chatterjee, Olaf Deutschmann and

Jurgen Warnatz(2001), Detailed surface reactionmechanism in a three way catalytic converter,

Interdisciplinary Centre of Scientific Computing

(IWR), Heidelberg University, 2001.

[4] Joachim Braun, Thomas Hauber, and JuliaWindmann(2004), Three-Dimensional Simulation

of the Transient Behaviour of a Three-Way

Catalytic Converter, SAE 2004-01-0148.

[5] Julia Windmann, Joachim Braun and PeterZacke(2003), Impact of the inlet flow distribution

on the light off behaviour of a 3-way catalytic

converter, SAE-2003-01-0937.

[6] G.Gaiser, J.Oesterie, and J.Barun(2003), The

progressive spin inlet-homogeneous flowdistributions under stringent conditions, SAE

2003-01-0840.

[7] Ming Chen, Joe Alexio, and ThierryLeprince(2004), CFD Modelling of 3-way catalytic

converter with detailed catalytic surface reaction

mechanism, SAE 2004-01-0148.

[8] Soojin Jeong and Taehun Kim(1997), CFD

investigation of the 3-Dimensional Unsteady flowin the catalytic converter, SAE 1997-971025.

[9] William Taylor III(1999), CFD Prediction andexperimental validation of high temperature

thermal behaviour in catalytic converters, SAE-1999-01-0454.

[10] FLUENT 6.1 help manual.

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Table 1: Boundary specification of catalytic converter

Table 2 : Results of CO and NO

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Figure 1: Dimensions of porous media

Figure 2 : Geometric modelling of catalytic converter

Figure 3: Meshed model of catalytic converter

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Figure 4: Pressure contour for a mass flow rate of 0.08 kg/s

Figure 5: Velocity Vectors

Figure 6: Mass flow V/S pressure drop curve

Figure 7: Experimental V/S computational curve

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Figure 8: Temperature distribution in catalytic converter at 700K