COMPUTATIONAL STUDY ON THE EFFECTS OF VOLUME RATIO OF DOC/DPF AND CATALYST LOADING ON THE PM AND NOX EMISSION CONTROL FOR HEAVY-DUTY DIESEL ENGINES

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1 International Journal of Automotive Technology, Vol. 9, No. 0, pp (2008) DOI /s Copyright 2008 KSAE /2008/04 01 COMPUTATIONAL STUDY ON THE EFFECTS OF VOLUME RATIO OF DOC/DPF AND CATALYST LOADING ON THE PM AND NOX EMISSION CONTROL FOR HEAVY-DUTY DIESEL ENGINES S. J. LEE 1), S. J. JEONG 2), W. S. KIM )* and C. B. LEE 4) 2) 1) Department of Mechanical Engineering, Graduate School of Hanyang University, Seoul 1-791, Korea Advanced Power & IT Research Center, Korea Automotive Technology Institute, 74 Yongjeong-ri, Pungse-myeon, Cheonan-si, Chungnam 0-912, Korea ) Department of Mechanical Engineering, Hanyang University, Gyeonggi , Korea 4) Environmental Parts R & D Center, Korea Automotive Technology Institute, 74 Yongjeong-ri, Pungse-myeon, Cheonan-si, Chungnam 0-912, Korea (Received 2 August 2007; Revised 28 July 2008) ABSTRACT The use of a diesel particulate filter (DPF) in a diesel aftertreatment system has proven to be an effective and efficient method for removing particulate matter (PM) in order to meet more stringent emission regulations without hurting engine performance. One of the favorable PM regeneration technologies is the NO 2 -assisted regeneration method due to the capability of continuous regeneration of PM under a much lower temperature than that of thermal regeneration. In the present study, the thermal behavior of the monolith during regeneration and the conversion efficiency of NO 2 from NO with an integrated exhaust system of a diesel oxidation catalyst (DOC) and DPF have been predicted by one-channel numerical simulation. The simulation results of the DOC, DPF, and integrated DOC-DPF models are compared with experimental data to verify the accuracy of the present model for the integrated DOC and DPF modeling. The effects of catalyst loading inside the DOC and the volume ratio between the DOC and DPF on the pressure drop, the conversion efficiency, and the oxidation rate of PM, have been numerically investigated. The results indicate that the case of the volume ratio of DOC/DPF=1.5 within the same diameter of both monoliths produced close to the maximum conversion efficiency and oxidation rate of PM. Under the engine operating condition of 175 kw at 2200 rpm, 100% load with a displacement of 8.1, approximately 55 g/ft of catalyst (Pt) loading inside the DOC with the active Pt surface of 5. m 2 /g pt was enough to maximize the conversion efficiency and oxidation rate of PM. KEY WORDS : NO 2 -assisted regeneration, DOC, DPF, Catalyst loading, Modeling NOMENCLATURE A cell : cross sectional area of the soot layer, m 2 a c : channel width, m c : molar concentration, mole/m D eff : effective diffusion coefficient, m 2 /s F : frictin coefficient (= for square channel) h : enthalpy, J/kg h : reaction enthalpy, J/mole k : permeability, m 2 m soot : local soot mass MG : molar mass, kg/mole p : pressure, (Pa) R : number of reactions R dep : rate of soot deposition, kg/s R reg : rate of soot regeneration, kg/s S : number of species *Corresponding author. wskim@hanyang.ac.kr t : time, sec T : temperature, K V cell : volume of the given calculation cell, m v : gas velocity, m/s w : mass fraction x : coordinate in wall thickness direction, m y : mole fraction z : coordinate in axial direction, m α : total channel surface area, m 2 /m ε : open frontal area ε L : soot layer porosity, m /m λ : heat conductivity, W/mK ν : stoichiometric coefficient µ : dynamic viscosity, Pas ρ : density, kg/m δ soot : soot height, m δ w : filter wall thickness, m SUBSCRIPTS i : reaction index, inlet (i=1), outlet (i=2) 1

2 2 S. J. LEE, S. J. JEONG, W. S. KIM and C. B. LEE j g L s w : species index : gas phase : solid surface layer : solid : wall 1. INTRODUCTION Developments in environmentally sound technologies are becoming more important for automakers competing in the world market. Japan, Europe, and the US plan to implement by 2010 drastically toughened restrictions on automobile emissions, with vehicles that fail to clear the standards banned from sale. Diesel-powered vehicles have about 20~0% higher fuel consumption efficiency and better durability than gasoline powered vehicles. From an environmental point of view, the advantages of diesel powered vehicles have also been positively recognized due to the relatively lower emissions of CO 2, CO, and THC than gasoline powered vehicles, resulting in the reduction of the greenhouse effect. However, diesel contains more sulfur, which means higher particulate matter (PM) emissions when fuel is burned incompletely at low temperatures, and they discharge large amounts of NOx when the fuel is burned completely at high temperatures. This means diesel exhaust emissions contain higher PM and NOx than gasoline-powered ones. PM and NOx are typical of harmful substances contained in exhaust gas. PM may cause respiratory problems in the human body, while NOx is the cause of photochemical smog and acid rain. There has been considerable effort to develop remarkable technologies that reduce emissions of both of NOx and PM, which has led to the DOC and DPF technologies. The DOC can reduce HC and CO emissions in the hot exhaust stream by 50% or more and the SOF (soluble organic fraction) of PM by up to 90% (Makoto et al., 1990). Control of the SOF enables the DOC to reduce total PM emissions by 25% to 50%, depending on the constituents that make up the total PM. In addition, the DOC plays a vital role in the generation of NO 2, which is a stronger oxidizing agent of PM than O 2, from the NO oxidation reaction in an integrated DOC-DPF system. The numerical analysis of the integrated DOC-DPF model provides valuable insight into the regeneration process of PM and could lead to efficient strategies involving thermal management of a PM control system. Since the early 1980s there have been plenty of modeling and numerical simulations to describe the operation of DPF and DOC technologies under typical diesel operating conditions. Oh and Catendish (1982) presented a one-dimensional model of a three-way catalytic converter that was applied to the DOC by Triana et al. (200). Voltz (197) studied the kinetics of CO and C H 6 on Pt catalysts and provided the reaction rate constants that are commonly used as a baseline to calibrate the DOC model using experimental data. Olsson et al. (1999) reported a kinetic model of the NO oxidation to NO 2 over a Pt/Al 2 O catalyst in the temperature range of 250 o C~450 o C. Bissett (1984) developed a steady-state isothermal flow model for the regeneration process of the DPF with a single-channel approximation. Opris (1997) developed a two-dimensional model of the flow, filtration characteristics, and physicochemical phenomena of the DPF. Konstandopoulos et al. (2000) presented an excellent review of the fundamentals of the most current DPF technology. The models take from simple analytically solvable pressure drop correlations over a broad variety of one-dimensional filter flow and regeneration models to three-dimensional models considering one representative pair of filter channels with CFD. Konstandopoulos et al. (2001) studied the effect of spatial non-uniformities on diesel particulate filters and investigated the effect of secondary inertial contributions. Zhang et al. (2002) developed a two-dimensional single-channel model to describe the loading and regeneration process of a DPF. Huynh et al. (200) considered a catalyzed wall-flow DPF using the two layer regeneration theory from Konstandopoulos et al. (2000) and the impact of depth and cake filtration regimes on the pressure drop during transient loading phases. Gaiser and Mucha (2004) investigated the influence of ash and different ash distributions on the filter pressure drop. Konstandopoulos et al. (200) extended a multi-channel model of a DPF by modifying the filter wall energy balance to include axial and radial components. These models have given a better understanding of the control parameters that define the filtration and regeneration characteristics of a DPF, and also the physicochemical characteristics of a DOC. However, most simulations from the previous research focused on one component, the DOC or DPF, and showed each individual effect from pragmatic approaches for filter flow, soot filtration, regeneration process, and conversion efficiency of species. Few works have reported the regeneration efficiency of a DPF as a function of the DOC volume and precious catalyst loading inside a DOC even though both of them are basic guidelines in the early stages of diesel emission system design. Therefore, a high efficiency integrated system of diesel aftertreatment is numerically analyzed in terms of geometric shape of the DOC/DPF volume ratio and precious catalyst loading of the DOC for NO 2 -assisted continuous regeneration in the present study. 2. MATHEMATICAL MODELING To predict the effects of the DOC/DPF volume ratio and precious catalyst loading inside the DOC on the conversion efficiency of species and the oxidation rate of PM, an integrated DOC-DPF model of a diesel aftertreatment system is employed in the present study, and shown in Figure 1. A general integrated DOC-DPF system can maximize the efficiency of the PM oxidation rate because plenty of NO 2 generated from the NO oxidation reaction inside the DOC is supplied into the DPF and is oxidized with PM. The diameter of both monoliths, the DOC and DPF, is fixed at 266 mm but the length of the DOC is varied while that of

3 COMPUTATIONAL STUDY ON THE EFFECTS OF VOLUME RATIO OF DOC/DPF AND CATALYST LOADING Figure 1. Integrated DOC-DPF model. Table 1. Specification of engine, DOC and DPF for the present study. Specification Engine Cylinder 6, in-line Displacement 8.1 l Rated power rpm Injection system High pressure common-rail Specification DOC DPF Material Cordierite Cordierite Diameter (mm) Length (mm) 0.5~ Wall thickness (mm) Cell density (cpsi) Densityw (kg/m ) Thermal conductivityw (W/mK) Specific heatw (J/kgK) the DPF remains constant for this work. The specifications of the engine, DOC, and DPF used in this study are summarized in Table Diesel Oxidation Catalyst The NO2 generated from the DOC enters the DPF and oxidizes PM. Hence the DOC is modeled to predict the NO2 upstream of the DPF. The predicted NO2 concentration is used as an input in the DPF model to estimate the oxidation rate of PM. A single-channel model is used to describe the thermal fluid flow and chemical reaction phenomena occurring inside the DOC as shown in Figure 2. The following primary assumptions are necessary for the singlechannel model used in this work: the flow characteristics, material properties, and reaction conversion are uniform across all the channels of the DOC; the thermal gradients in the radial direction are neglected; the outer wall in the radial direction of the DOC is completely insulated; the washcoat layer is so thin compared to the wall thickness that the temperature of the washcoat and that of the substrate are the same; the catalytic reaction occurs on the surface of the washcoat. The material and energy balances of the gas phase in a channel can be described as follows: (2) Diffusion inside a channel is usually small compared to convection and radial transport at the typical space velocities for automotive exhaust aftertreatment but becomes important for small Pe of mass transfer. Hence the diffusion term in equation (1) is included for the generality of the simulation due to the relationship between the magnitude of Pe and the directionality of influence. Assuming that the external transport resistances are the dominant ones, the energy balance for the solid phase is expressed by: Due to the chemical reactions occurring on the surface of a catalyst, the concentrations of the species directly above the catalytic surface are not equal to the concentration of species in the bulk. This effect is accounted for by solving additional balance equations for the individual species concentrations at the solid surface. Therefore it is possible to take into account the two cases of chemical and mass transfer limitations. Under the assumption of quasi-steady state conditions, the rates of the catalytic surface reactions balance the diffusive transport from the bulk gas to the surface. Thus, the molar surface concentration can be evaluated using: (1) (2) () (4) Figure 2. Schematic of a single-channel in a honeycomb type DOC. Assuming laminar flow in the square channel, the pressure drop across the DOC is described by Darcy s law

4 4 S. J. LEE, S. J. JEONG, W. S. KIM and C. B. LEE Table 2. Heat and mass transfer coefficients. k h =Nu λ g /d hyd k k, m =Sh D k, g /d hyd Nu=1.86 ( Gz heat ) 1/ Sh=1.86 ( Gz mass ) 1/ Gz heat =Re Pr d/l Gz mass =Re Sc d/l where λ g is the thermal conductivity of the exhaust gas, D k,g is the diffusion coefficient of species k, d is the hydraulic diameter of the channel, and l is the channel length. (Kaviani, 1991). The transport coefficients for heat, k h, and species mass, k k,m, in equations (2)~(4) are estimated based on empirical relations for Nu and Sh. The Sieder-Tate approach (Perry and Green, 1984) is adopted to obtain the analytical solutions of Nu and Sh using Gz, for fully developed laminar flow. Table 2 shows the summary of this approach. The ideal gas equation of state is used to calculate the density of the exhaust gas. According to procedures and data from Verein Deutscher Ingenieure (1991) and Reid and Prausnitz (1987), the viscosity, diffusion coefficient, and thermal conductivity are calculated as a function of temperature, pressure, and composition of exhaust gas. The thermodynamic data of the exhaust gas are estimated as a function of the temperature and composition of exhaust gas (Barin, 1985). Under the assumptions that a catalytic reaction occurs uniformly across the surface and products are weakly bound and rapidly desorbed, the Langmuir-Hinshelwood kinetic approach is employed for the DOC modeling to represent heterogeneous reactions because adsorption effects on the surface of the washcoat are important. In the present study, the following oxidation reactions over Pt are considered to take place: 1 R1 : CO+ -- O 2 CO 2 (6) 2 1 R2 : C H O 2 CO 2 +H 2 O (7) 2 1 R : NO+ -- O 2 NO 2 (8) 2 The species reaction rate of reaction R1~R are given below: (5) Table. Apparent activation temperatures and preexponential factors. Engine Speed: 2200 RPM CO Temperature range ( o C) Ea/R (K) A (molk/m 2 s) 290 < T E < T < E+06 T < E+11 HC Temperature range ( o C) Ea/R (K) A (molk/m 2 s) 290 < T E < T < E+07 T < E+09 NO Temperature range ( o C) Ea/R (K) A (molk/m 2 s) 280 < T E < T < E+14 T < E+10 where G represents an inhibition factor, Y j is molar fraction of species i and the reaction rate constant k j is assumed to follow an Arrhenius form as: k j =A exp( E a /RT) (1) The apparent pre-exponential factors and activation temperatures of CO, HC, and NO used in the present study are selected by Triana (2005) based on the best linear regression models for each temperature range and engine speed condition. Table shows the summary of the apparent preexponential factors and activation temperatures Diesel Particulate Filter Model review Figure shows the single-channel model of the general wall-flow DPF used in this study. Along the inlet and outlet channels of the DPF, the single-channel governing equations for unsteady, compressible flows are solved. The governing equations describing thermal fluid flow and (9) (10) (11) (12) Figure. Schematic of a single inlet and outlet channel with the porous wall and the deposited soot layer.

5 COMPUTATIONAL STUDY ON THE EFFECTS OF VOLUME RATIO OF DOC/DPF AND CATALYST LOADING 5 chemical reaction phenomena during regeneration can be simplified by considering several assumptions. The thermodynamic properties of exhaust gas entering the front face of the DPF are quantitatively averaged over the cross section of inlet channel due to the spatially uniform flow, but possibly time dependent. All pairs of inlet and outlet channels can be represented by a single channel model as shown in Figure. The gas phase process is assumed to be quasisteady state due to the short residence time of exhaust gas in the inlet and outlet channels of the DPF. The ideal gas equation of state is assumed for the exhaust gas, as is fully developed laminar flow in the inlet and outlet channels. Additional assumptions are that the outer wall in the radial direction of the DPF is completely insulated; conduction is dominant in the radial direction of the DPF; carbon particulate matter is piled up with uniform thickness in the axial direction of the monolith. For an internal channel flow of the DPF, an axial gas velocity is dominant compared to the radial velocity because the channel length is several orders of magnitude larger than channel width (Peters et al., 2004). This allows the flow could be treated as one-dimensional channel flow. The z-component of momentum in the porous wall is neglected because the transport of momentum perpendicular to the x-direction in the porous wall is only a small fraction of the wall-flow. The following thermal and NO 2 - assisted regeneration reactions of carbon are taken into account in this study. R4 : 2C+O 2 2CO (14) R5 : C+O 2 CO 2 (15) R6 : C+NO 2 CO+NO (16) The corresponding reaction rates for above reactions R4~ R6 are defined in equations (17)~(19), respectively and the kinetic parameters used in this study are from Konstandopouls work (2000) for thermal regeneration and from Triana s work (2005) for NO 2 -assisted regeneration. The activation energy, E, and frequency factor, K, used in equations (17)~(19) are key factors that represent the PM oxidation pattern during regeneration of the DPF. Moreover, the accuracy of computational results is heavily dependent on the value of these factors. In order to consider an effect of thermal and NO 2 -assisted regeneration, the modified activation energy, K 4 ~K 6, and frequency factor, E 4 ~E 6, are employed in the present DPF model due to the difference in the form of reaction rate, r 4~r 6, from the literature (Triana, 2005; Konstandopoulos and Kostoglou, 2000). Thermal CO selectivity for carbon oxidation, f th CO, is given in equation (20). E r 6=K exp y RT NO2 (19) f th CO =1+k f y q e E f ( /RT ) 1 (20) The numerical values of kinetic parameters used in the present study are listed in Table 4. The governing equations (1)~() for the energy balance and species conservation of the solid and gas phases are also solved for the DPF. The conservation of mass equations for an inlet channel with a variable cross section and an outlet channel are described as follows: (21) (22) The conservation of momentum equations are written as follows: (2) (24) Due to the low velocity, the transport of momentum perpendicular to the main direction of the flow is neglected. The shear stress exerted on the flow by friction along the channel walls is taken into account through a linear correlation between loss coefficient F and the local channel velocity. This represents the integral effect of the shear transverse to the flow direction. The mass conservation of PM at any position in a channel is described by the following equation, m =R soot dep R reg (25) t where R dep and R reg represent the rate of PM deposition and PM regeneration, respectively. A detailed explanation of the PM deposition process used in the present study is given in the literature (Peters et al., 2004). The continuity and transport equations for the mass fractions of species in the PM layer are given in equations (26) and (27), respectively. The source terms included in those equations are shown in equations (28) and (29). (26) (27) (28) E 1 r 4=f CO K 1 exp y RT O2 E r 5= ( 1 f CO ) K 2 exp y RT O2 (17) (18) (29) Where S j is the source term of the species j due to the heterogeneous reactions and S tot is the summation of the

6 6 S. J. LEE, S. J. JEONG, W. S. KIM and C. B. LEE source terms of all heterogeneous regeneration reactions. The rate of PM regeneration can be calculated as follows: (0) where, x=0 and x=δ soot represent the boundary between the PM deposit layer and the substrate wall, and between the PM deposit layer and the exhaust gas, respectively. The governing equations for the DOC and the DPF are solved by a commercial CFD code developed by AVL, Boost (2005), where user-subroutines are linked with Boost s main code to implement chemical reactions for the DOC and to calculate the kinetic reaction rates and catalyst distribution function Pressure drop model The substrate wall and the PM deposit layer on the filter can be treated as different two porous media in series. Since the pressure variations in the radial direction are negligible, Darcy's law (Kaviani, 1991) is used to describe the respective pressure drop between the inlet and outlet channels at any location along the DPF length. (1) The wall velocity v w (x) is derived from continuity through the soot layer in the following form: (2) Integration of equation (1) yields the following expression for the pressure drop: Table 4. Specification of the DPF and initial conditions used in DPF model validation (Miyairi et al., 2001), and kinetic parameters used in the present study. Engine (Soot created by a diesel fuel burner) DPF specification Material SIC Diameter (mm) 144 Length (mm) 152 Wall thickness (mm) 0.05 Cell density (cpsi) 00 Density w (kg/m ) 1700 Thermal conductivity w (W/mK) 1 Specific heat w (J/kgK) 1000 Permeability w (m 2 ) Permeabilitys (m 2 ) Soot layer packing density (kg/m ) 100 Initial conditions Exhaust gas mass flow rate (kg/s) Exhaust gas temperature ( o C) 600 O 2 concentration (%) 10 Initial PM deposited (g) 10 Kinetic parameters Pre-exponential factor (1/s) Activation energy (kj/kmol) 4.50E+07 (for O 2 ) 1.50E+08 (for NO 2 ) (for O 2 ) 7000 (for NO 2 ) () 2.. Initial and Boundary Conditions To solve the above differential equations, several initial and boundary conditions for the system must be specified. The initial and boundary conditions for the integrated DOC- DPF model used in the present study are obtained from experimental and simulation data presented by Triana s work (2005) and summarized in Table 5. The following boundary conditions of DPF are applied: v 2 ( 0 )=0 p 2 ( L )=p atm. MODEL VALIDATION (4) (5).1. Case 1: DPF Model The experimental results of Miyairi et al. (2001) are employed for the validation of the pure thermal regeneration technique to show a basic prediction performance of thermal regeneration behavior under steady-state engine operating conditions. The specifications of the DPF, initial conditions, and kinetic parameters used in the DPF model validation are summarized in Table 4. Figure 4 shows the comparison of the measured results from the literature (Miyari et al., 2001) and the ones calculated from our model for the temporal evolution of wall temperature at the 5 points on the DPF center axis. In the experiment, the employed SiC DPF is a 144 mm(d) 152 mm(l) with a Table 5. Input data used in the model. Speed (RPM) Load (%) T g in T w in Q gas C in O 2 o C o C actual m /s actual m /s % vol CO 2 H 2 O CO NO NO 2 HC N 2 % vol. % vol. ppm ppm ppm ppmc % vol Balanced

7 COMPUTATIONAL STUDY ON THE EFFECTS OF VOLUME RATIO OF DOC/DPF AND CATALYST LOADING 7 Figure 4. Comparison of the calculated and measured wall temperatures for the SiC DPF. Figure 5. Conversions of CO, HC, and NO at 2200 rpm: (a) experimental results from Triana; (b) present simulation results. cell density of 00 cpsi and wall thickness of 0.05 mm. A gas temperature of 600 o C and a flow rate of kg/s is used for the PM regeneration. After warm-up, the heat from convective heat transfer of the hot exhaust gas and PM oxidation inside the DPF is continuously accumulated on the front side of the DPF. Consequentially, the increase of gas temperature may be accelerated and transferred to the rear side of the DPF. This leads to the increase of the DPF temperature and causes the temperature of the DPF to rise beyond that of the exhaust gas. The calculated results are in agreement with the measured ones, within the error ranges of about % and 1% for the maximum regeneration temperature and the light-off time, respectively..2. Case 2: DOC Model Figure 5 compares the DOC conversion rates of the experimental results of Triana (2005) and that of the presented simulation to validate our DOC model. Triana conducted a series of experiments at several engine operating conditions, 5~100% load, 1400~2200 RPM with a 6-cylinder inline, turbocharged 8.1 diesel engine, to predict the effects of an integral DOC-DPF system on the regeneration efficiency of the DPF. Conversion rates of CO, HC, and NO obtained from the experiments of 10~100% load at 2200 RPM (Triana, 2005) are compared with results from our simulation. The operating conditions at each of the steadystate engine speeds and the specific exhaust gas data used in this work are reported in more detail in the literature (Triana, 2005). The controlling rate step is temperature dependant, causing the apparent activation energies and pre-exponential factors to be different. Therefore, Triana did not use the average kinetic parameters obtained from the model calibration at each of the steady-state conditions, but instead used kinetic parameters that corresponded to the temperature of exhaust gas in the simulations to compare with the experiments (Triana, 2005). The pre-exponential factors used in this model are slightly modified from the data used by Triana (2005) because there is no information on the precious catalyst loading in the literature. Figure 5 shows the results of the conversion rate in the DOC obtained under this assumption. The purpose of this validation is to evaluate the NO to NO 2 conversion rate to determine an accurate NO 2 concentration, which will then be used in the PM regeneration process inside the DPF. The conversion rates show a strong dependence on the temperature of exhaust gas. The results show that the DOC completely equilibrates the gas phase at temperatures above 0 o C, and hence the conversion rate decreases at temperatures above 0 o C because of thermodynamic limitations. The NO 2 concentration is kinetically controlled at lower temperatures because the activity of the catalyst is not sufficient to convert enough NO to reach thermodynamic equilibrium. Platinum based catalysts are not active enough to facilitate enough oxidation of NO to NO 2 at typical space velocities for automotive exhaust aftertreatment at temperatures below 150 o C. The slight discrepancy between experimental and simulation results in the temperature range of 00 o C < T < 70 o C occurs due to the uncertainty of precious catalyst information mentioned above. However, the simulation results are overall similar to the experimental results. The model s calculation of the conversion rate as a function of gas temperature is in agreement with past work in the literature (Triana, 2005). The comparison shows that the predicted conversion rate of NO is more accurate than that of CO or HC. The results also imply that the DOC model used is suitable for NO 2 -assisted regeneration in the integrated DOC-DPF model... Case : Integrated DOC-DPF Model Figure 6 shows the comparison between the model results

8 8 S. J. LEE, S. J. JEONG, W. S. KIM and C. B. LEE 2200 rpm, 25~75% load were conducted to demonstrate the model capabilities and these results are also compared with that of experiments (Triana, 2005). Similarities for both the pressure drop and PM loading are exhibited, and a stable deviation of the changing rate is shown in the comparison even though some degree of error is generated. These results demonstrate the ability of the integrated DOC- DPF model to reliably predict the various physicochemical phenomena occurring inside the DOC and the DPF. 4. RESULTS AND DISCUSSION Figure 6. Comparison of the DPF pressure drop and the PM deposits between DPF and integrated DOC-DPF model at 2200 rpm, 100% load; 00 cpsi DOC, and 200 cpsi DPF. and the experimental results from the literature (Triana, 2005) for the DPF pressure drop and PM loading as function of time. The experiment was conducted at 100% load at 2200 rpm with 00 cpsi DOC and 200 cpsi DPF. The comparison shows that four hours after the experiment begins, the pressure drop across the DPF and PM loading inside the DPF obtained from the integrated DOC-DPF model is overestimated by about 12% and 40%, respectively. Since there is no exact input data for the integrated DOC-DPF model in the literature, the deviation between predictions and experiments is attributed to the uncertainty of species concentrations upstream of the DOC and the associated uncertainty of the reaction kinetics. The input data for the DOC model had to be used in the integrated DOC-DPF model. However, the additional simulations at The object of this study is to predict the effects of the DOC/ DPF volume ratio and the precious catalyst loading inside the DOC on the efficiency of conversion rates and PM regeneration. The integrated DOC-DPF model is used to supply the exhaust gas data at the outlet of the DOC that is used as the input data to the DPF. Identical operating conditions as in the experiment by Triana (2005), 100% load at 2200 RPM, were assumed in this work as shown in Table 5. Steady-state results are obtained an hour after beginning the simulation. Detailed explanations for the effects of the DOC/DPF volume ratio and the precious catalyst loading inside the DOC on the efficiency of conversion rates and PM regeneration are given in the following section Effects of DOC/DPF Volume Ratio The concentration of NO 2 is evaluated at the outlet of the DOC according to a DOC/DPF volume ratio because NO 2 entering the DPF is a more effective oxidant for the PM regeneration than O 2 at temperatures greater than about 00 o C. The main focus of this section is on the prediction of the DPF regeneration efficiency and pressure drop for Table 6. Summary of simulation cases for the dimensions of the DOC and DPF used in the model. Case no. DOC (00 cpsi) Volume DPF (200 cpsi) Diameter (m) Length (m) Volume (m ratio ) Diameter (m) Length (m) Volume (m ) : : : : : : : : : : : : : :

9 COMPUTATIONAL STUDY ON THE EFFECTS OF VOLUME RATIO OF DOC/DPF AND CATALYST LOADING 9 increasing DOC volume and constant DPF volume. However, this process also affects the conversion efficiency of species in the DOC. The summary of simulation predicting the effects of the DOC/DPF volume ratio on the efficiency of conversion rates and PM regeneration are reported in Table 6. Figure 7 shows the evolution of conversion efficiency as function of DOC/DPF volume ratio. The conversion efficiency of CO and HC increases sharply at the lower DOC/ DPF volume ratios. This due to the increased surface area of the reaction inside the DOC when increasing the DOC volume while holding the DPF volume constant. The increasing rates of CO and HC conversion efficiencies are smoothly decreased as the DOC/DPF volume ratio approaches 1. The increasing rates of NO conversion efficiency are increased faster than either CO or HC as the DOC/DPF volume ratio is increased until DOC/DPF>1.5. However, the magnitude of the conversion efficiency of NO is relatively lower than that of CO and HC as a result of the reaction rates at the operating conditions. The NO conversion efficiency shows an almost linear increase until the DOC/DPF volume ratio approaches 1. The increasing rate of the NO conversion efficiency also smoothly decreases as the DOC/DPF volume ratio approaches 1.5. No significant differences in the conversion efficiency of species are shown in cases where the DOC/DPF>1.5. Figure 8 presents the evolution of pressure drop across the DOC and the DPF as a function of the DOC/DPF volume ratio. The pressure drop of the DPF is generally more severe than that of the DOC. When the length of the DPF having the same diameter is decreased or increased, the flow rate per unit area of exhaust gas passing through the porous wall is increased or decreased. Therefore, the total pressure drop in the system may increase in the cases of the longer DOC or the more shortened DPF than for both monoliths having the standard geometry based on the Figure 7. Conversion efficiency of the DOC as a function of the DOC/DPF volume ratio. Figure 8. Pressure drop in the DOC and the DPF as a function of the DOC/DPF volume ratio. Figure 9. PM mass in the DPF and mole fraction of NO 2 and NO as a function of the DOC/DPF volume ratio. volume ratio of DOC/ DPF=1. Since the DPF dimensions used in the present study are held constant while varying the length of the DOC having the same diameter, there is little change in the pressure drop of the DPF for all cases. The pressure drop of the DOC linearly increases as the volume ratio of the DOC/ DPF is increased, almost reaching the value of the DPF pressure drop in the case of the volume ratio of DOC/ DPF=2. Figure 9 depicts the remaining PM mass inside the DPF and the mole fractions of NO and NO 2 obtained from downstream the DOC as a function of the DOC/DPF volume ratio. The remaining PM mass inside the DPF is directly influenced by the efficiency of PM regeneration. When the mole fraction of NO 2 is increased in the cases of DOC/DPF volume ratios less than about 1, the accumulated PM is linearly depleted. As the DOC/DPF volume

10 10 S. J. LEE, S. J. JEONG, W. S. KIM and C. B. LEE ratio approaches 1.5, the decreasing rate of accumulated PM is smoothly decreased because the conversion efficiency of NO to NO 2 is almost saturated near the volume ratio of DOC/ DPF=1.5. It can be noted that the increased NO 2 due to the high conversion efficiency caused by an increase in the volume ratio of DOC/DPF is the critical factor for the efficiency of PM regeneration. In this study, the phenomenon of NO 2 -assisted regeneration is effectively described as the DOC/DPF volume ratio is increased. It can also be observed that the efficiency of PM regeneration almost reaches the maximum in the case of DOC/ DPF Effects of Precious Catalyst Loading No catalyst data is mentioned in the reference (Triana, 2005) because it is proprietary. The Pt/Al 2 O system was used for the DOC in the present study because of the available catalyst data in the literature (Marcus et al., 2005). The active Pt surface is calculated to be 5. m 2 /g pt by equation (6). A act [ m 2 /g pt ]= V ads N A n a m 100 (6) V m m w where, A act is the active Pt surface area, V ads is the volume of chemisorbed CO, V m is the molar volume, N A is Avogadro s number, m is the mass of catalyst and w is the weight percentage of Pt. The CO:Pt stoichiometry, n, is assumed to be 0.7 (Loeoef et al., 1991) and the surface area per Pt site, a m, is taken as 8.07 Å 2 (Bergeret and Gallezot, 1997). Based on the above active Pt surface for the DOC, the various loadings of precious catalyst, 2~80 g Pt /ft monolith, are used in the present study. The catalyst distribution function of Pt, 2 a(z) [ m Pt /m monolith ], represents the catalytic surface area per unit reactor volume and can instead be expressed in terms Figure 10. Conversion efficiency of the DOC as a function of Pt loading. Figure 11. Pressure drop and PM mass in the DPF as a function of Pt loading. of the catalytic mass per unit reactor volume, M[ g Pt /ft monolith ], as shown in equation (7). a( z) M[ g Pt /ft monolith ]= (7) A act Figure 10 shows the conversion efficiency of species inside the DOC as a function of precious catalyst loading. The similar patterns of variation in conversion efficiency for CO, HC, and NO are observed in the simulations. The conversion efficiency of species is rapidly increased as Pt loading is increased in the condition of Pt loading < 27 g Pt /ft monolith, up to 90% of the conversion efficiency for the all species in the present study. The conversion efficiency of NO increases considerably more than that of either CO or HC for Pt loading < 27 g Pt /ft monolith. Almost no conversion efficiency variations are shown for the condition of Pt loading > 55 g Pt /ft monolith, where they are greater than 95% for all species. Figure 11 presents the pressure drop across the DPF and the remaining PM mass inside the DPF as a function of precious catalyst loading. The increase of Pt loading inside the DOC contributes to the improvement of the conversion efficiency of species, resulting in the increase of NO 2 concentration upstream of the DPF and in the acceleration of NO 2 -assisted PM oxidation. The rapid depletion of PM mass is also observed as Pt loading is increased for Pt loading < 27 g Pt /ft monolith, but the effect of the increase in Pt loading is gradually decreased for Pt loading > 27 g Pt /ft monolith. There is almost no variation of the remaining PM mass in the condition of Pt loading > 55 g Pt /ft monolith. As a result, approximately 0~21% improvement of regeneration efficiency is achieved as Pt loading is increased from 2 to 80 g Pt /ft monolith for the operating conditions explored. The pressure drop across the DPF is affected by the variation of accumulated PM mass. The pressure drop

11 COMPUTATIONAL STUDY ON THE EFFECTS OF VOLUME RATIO OF DOC/DPF AND CATALYST LOADING 11 variation shows a similar tendency with that of the remaining PM mass inside the DPF, but the rate of variation is negligible for the operating conditions of interest. The results indicate that the optimum precious catalyst loading should be predicted according to the various driving conditions in the early stage of system design to select a minimum precious catalyst loading without inhibiting system performance. 5. CONCLUSIONS The effects of DOC/DPF volume ratio and precious catalyst loading inside the DOC are numerically predicted in the present work to apply the NO 2 -assisted continuous regeneration method in the early stage of diesel emission system design. The developed model demonstrates the great potential of the NO 2 -assisted regeneration technique in evaluating kinetic and mechanistic aspects of the integrated DOC-DPF system. Recall that our discussion so far has been limited to the case where the engine used in this study is 8.1 l at 2200 rpm, 100% load for 00 cpsi DOC, and 200 cpsi DPF with equivalent diameters. However, the present numerical model will play a key role in the acceptance of the high efficiency multi-combined emission system in future work and the present results could be useful to other integrated aftertreatment systems. The results in the present study can be summarized as follows: (1) Increases to the DOC while holding the DPF volume constant results in an increase in the conversion efficiencies of species due to the increased surface area of reaction inside the DOC. Conversion efficiencies of over 94% for all species are produced in the cases up to DOC/DPF=1.5. No significant differences in the conversion efficiencies are shown for DOC/DPF>1.5. (2) When the mole fraction of NO 2 is increased in the cases of the DOC/DPF volume ratio of less than about 1, the accumulated PM is depleted approximately linearly. As the DOC/DPF volume ratio approaches 1.5, the decreasing rate of accumulated PM smoothly decreases because the conversion efficiency of NO to NO 2 is almost saturated near the volume ratio of DOC/DPF =1.5. It was observed that the efficiency of PM regeneration approaches the maximum for DOC/DPF1.5. () Species conversion efficiency rapidly increased as Pt loading increased for Pt loading < 27 g Pt /ft monolith, up to 90% conversion efficiency for the all species in the present study. Almost no variations of the conversion efficiencies are shown for Pt loading > 55 g Pt /ft monolith which resulted in conversion efficiencies of over 95% for all species. (4) To select a minimum precious catalyst loading without system inhibition, the optimum precious catalyst loading should be predicted according to the various driving conditions in the initial stage of system design. Approximately 55 g Pt /ft monolith of catalyst (Pt) loading under the studied engine operating conditions is enough to maximize the conversion and regeneration efficiency. About 21% improvement of the PM regeneration efficiency is achieved with 55 g Pt /ft monolith Pt loading for the tested operating conditions. ACKNOWLEDGEMENT This study has been financially supported by the Korea Ministry of Commerce, Industry and Energy and Korea Automotive Technology Institute (KATECH) in the program of Mid and long term technological development, Project Grant No REFERENCES AVL Boost Ver (2005). Manual. AVL GMBH. Barin, I. (1997). Thermochemical Data of Pure SubstancesU rd Edn., John Wiley & Sons. New York. Bergeret, G. and Gallezot, P. (1997). Handbook of Heterogeneous Catalysis. John Wiley & Sons. New York. Bissett, E. J. (1984). Mathematical model of the thermal regeneration of a wall-flow monolith diesel particulate filter. Chemical Engineering Science, 9, Gaiser, G. and Mucha, P. (2004). Prediction of pressure drop in diesel particulate filters considering ash deposit and partial regeneration. SAE Paper No Huynh, C. T., Johnson, J. H., Yang, S. L., Bagley, S. T. and Warner, J. R. (200). A one dimensional computational model for studying the filtration and regeneration characteristics of a catalyzed wall-flow diesel particulate filter. SAE Paper No Kaviani, M. (1991). Principles of Heat Transfer in Porous Media. Mechanical Engineering Series. Springer. Berlin Heidelberg. New York. Konstandopoulos, A. G., Kostoglou, M. and Housiada, P. (2001). Spatial non-uniformities in diesel particulate trap regeneration. SAE Paper No Konstandopoulos, A. G., Kostoglou, M., Housiada, P., Vlachos, N. and Zarvalis, D. (200). Multichannel simulation of soot oxidation in diesel particulate filters. SAE Paper No Konstandopoulos, A. G., Kostoglou, M., Skaperdas, E., Papaioannou, E., Zarvalis, D. and Kladopoulou, E. (2000). Fundamental studies of diesel particulate filters: Transient loading, regenerations and aging. SAE Paper No Konstandopoulos, A. G. and Kostoglou, M. (2000). Reciprocation flow regeneration of soot filters. Combustion and Flame, 121, Loeoef, P., Kasemo, B. and Andersson, S. (1991). Influence of ceria on the interaction of CO and NO with highly dispersed Pt and Rh. J. Catalysis, 10, Makoto, H., Koichi, S. and Shoichi, I. (1990). The effects of flow-through type oxidation catalysts on the particulate reduction of 1990s diesel engines. SAE Paper No Marcus, C., Sven, K. and Werner, W. (2005). Mean field modeling of NO oxidation over Pt/Al 2 O catalyst under

12 12 S. J. LEE, S. J. JEONG, W. S. KIM and C. B. LEE oxygen-rich conditions. J. Catalysis, 229, Miyairi, Y., Miwa, S., Abe, F., Xu, Z. and Nakasuji, Y. (2001). Numerical study on forced regeneration of wall flow diesel particulate filters. SAE Paper No Oh, H. and Cavendish, J. C. (1982). Transients of monolithic catalytic converters: Response to step changes in feedstream temperature as related to controlling automobile emissions. Industrial and Engineering Chemistry Product Research and Development, 21, Olsson, L., Westberg, B., Persson, H., Fredell, E., Skoglundh, M. and Andersson, B. (1999). A kinetic study of oxygen adsorption/desorption and NO oxidation over Pr/Al 2 O catalysts. J. Physical Chemistry B, 10, Opris C. N. (1997). A Computational Model Based on the Flow, Filtration, Heat Transfer and Reaction Kinetics Theory in a Porous Ceramic Diesel Particulate Trap. Ph. D. Dissertation. Michigan Technological University. USA. Perry, R. and Green, D. (1984). Perry's Chemical Engineer's Handbook. 6th Edn., McGraw Hill. New Jersey. Peters, B. J., Wanker, R. J., Munzer, A. and Wurzenberger, J. C. (2004). Integrated 1D to D simulation workflow of exhaust aftertreatment devices. SAE Paper No Reid, R. and Prausnitz, J. M. (1987). Thermal Properties of Gases and Liquids. 4th Edn.U McGraw Hill. New Jersey. Triana, A. P. (2005). Development of Models to Study the Emissions, Flow, Kinetic Characteristics Form a Diesel Oxidation Catalyst and Particulate Filter. Ph.D. Dissertation. Michigan Technological University. USA. Triana, A. P., Johnson, J. H., Yang, S. L. and Baumgard, K. J. (200). An experimental and numerical study of the performance characteristics of the diesel oxidation catalyst in a continuously regenerating diesel particulate filter. SAE Paper No Verein Deutscher Ingenieure, Editor (1991). VDI-Warmeatlas, Berechnungsblatter Fur Den Warmeubergang. 6th Edn.. VDI-Verlag. Dusseldorf. Voltz, E. V., Morgan, C. R., Liederman, D. and Jacob, S. M. (197). Kinetic study of carbon monoxide and propylene oxidation on platinum catalysts. Industrial and Engineering Chemistry Product Research and Development, 12, Zhang, Z., Yang, S. L. and Johnson, J. H. (2002). Modeling and numerical simulation of diesel particulate trap performance during lading and regeneration. SAE Paper No