Electrically Heated Catalysts for Hybrid Applications: Mathematical

ARTICLE pubs.acs.org/IECR

Electrically Heated Catalysts for Hybrid Applications: Mathematical Modeling and Analysis Karthik Ramanathan,*,† Se H. Oh,‡ and Edward J. Bissett^,§ †

India Science Lab, General Motors Global Research and Development Center, Bangalore, Karnataka, 560066, India Chemical Sciences and Materials Systems Lab, General Motors Global Research and Development Center, Warren, Michigan 48090, United States § General Motors Global Research and Development Center, Warren, Michigan 48090, United States ‡

ABSTRACT: In view of the significant cold-start hydrocarbon emission reduction potential of the electrically heated converter (EHC) technology for conventional stoichiometric gasoline engines, there is considerable interest in better understanding of the thermal and emission performance characteristics and optimizing the design/operating aspects of an EHC system as applied to plugin hybrid electric vehicles (PHEVs) and extended-range electric vehicles (EREVs). The application of the EHC technology to these hybrid vehicles is unique in that catalyst cooling to below reaction temperatures can occur during extended periods of electric vehicle driving (with engine off) or during intermittent engine stops/starts, and the EHC can be heated prior to engine start (preheating) for enhanced emission reduction. In this study, the design aspects and heating strategies of an EHC system have been analyzed using a transient monolith converter model which accounts for the resistive heating of an inert metalsubstrate monolith placed ahead of a conventional three-way catalytic converter. The results of model calculations presented here quantify the effects of various heating strategies on the emission performance of hybrid vehicles during the first 250 s of the Federal Test Procedure (FTP) drive cycle. It is also shown that there exists an optimum electric heater volume for cases with either preheating only or a combination of pre- and postheating. For the latter case, the emission performance can be further improved by adding a smaller electric heater (downstream of the existing heater) which is capable of heating the gas rapidly and efficiently during postheating.

’ INTRODUCTION As part of the continuing drive toward near-zero vehicle emissions, another round of stricter exhaust emission regulations is expected to be in place by the middle of this decade, including the fleet-average SULEV (super ultra-low emission vehicle) emission requirement in California starting in 2014. The SULEV standards are very stringent particularly with respect to allowed tailpipe hydrocarbon (HC) emission levels, mandating a 9-fold reduction from the current Tier 2 Bin 5 HC standard. Since a large fraction (typically >80% for late model gasoline vehicles) of the total exhaust emissions of HC and CO occurs during the first few minutes after an engine cold start, it is crucial to further shorten the time required for an exhaust catalyst to reach its operating temperature in order to meet the stringent future emission control requirements. Strategies for reducing cold-start emissions include electrically heated catalysts,110 fuel-burner heated catalysts,11,12 hydrocarbon adsorbers,1317 exhaust gas ignitors,18 and energy storage devices.19,20 Electrically heated converter (EHC) technology is designed to heat the incoming exhaust gas via resistive heating of a metal-substrate monolith catalyst (mounted ahead of a conventional ceramic-substrate catalytic converter) using electrical power drawn from a vehicle battery or alternator. This technology has been extensively investigated for the control of cold-start HC emissions from conventional stoichiometric gasoline engines, addressing topics such as electrical power/energy reduction strategies, determination of the best size regime for the electric heater, and optimization of EHC system configuration. Although the emission benefit of using converter preheating (i.e., electrical heating prior to an engine start) in combination with postcrank heating has been demonstrated,2,3,7 previous studies on this r 2011 American Chemical Society

application focused primarily on postcrank electrical heating in view of potential customer acceptance problems associated with the waiting period required for converter preheating. For cases with postcrank heating only, it has been shown that for a given electrical power level, decreasing the volume of an electric heater generally improves the emission performance of the EHC system.4,7,9 Bissett and Oh10 analyzed the EHC sizing issue in detail and showed that with a sufficiently small electric heater, almost all of the supplied electrical energy is utilized to raise the temperature of the gas passing through the EHC, resulting in efficient heating of the main converter located downstream to the required reaction temperature. In addition, this main converter activation process can be further enhanced by increasing the noble metal loading in the upstream section of the main converter.9 This provides a rationale behind the development of optimized EHC system configurations which consist of a smallvolume (3550 cm3) electric heater followed by an unheated “lightoff” catalyst containing high concentrations of noble metals.5,8 Despite the progress made in EHC system development, EHC technology has not yet been used for widespread production application because improvement in catalyst thermal durability enabled the automobile manufacturers to meet the HC emission standards by using close-coupled catalysts with high noble metal loading in combination with engine-based catalyst heating strategies. Received: January 7, 2011 Accepted: April 1, 2011 Revised: March 11, 2011 Published: June 08, 2011 8444

dx.doi.org/10.1021/ie200044w | Ind. Eng. Chem. Res. 2011, 50, 8444–8467

Industrial & Engineering Chemistry Research Driven by the need for fuel economy improvement, plug-in hybrid electric vehicles (PHEVs) and extended-range electric vehicles (EREVs) are being developed as part of GM’s advanced propulsion technology strategy to increase fuel efficiency via vehicle electrification. The typical operating modes of these hybrid vehicles involve intermittent engine stops/starts, and this presents a significant challenge with respect to HC emission control due to catalyst cooling during extended periods of “allelectric” driving (i.e., vehicle powered by the battery with the internal combustion engine off). This raises renewed interest in EHC technology for potential application to hybrid vehicles for HC emission control. This is different from the conventional EHC application discussed above in two respects. First, electrical heating is no longer limited to a cold-start period because the catalyst bed can be cooled to temperatures below its light-off temperature during extended engine-off periods (e.g., longer than several minutes). Second, unlike the conventional EHC application, hybrid vehicles afford an opportunity to achieve enhanced HC emission reduction by using converter preheating (in addition to postheating) without customer acceptance problems. In fact, as our simulation results will show, catalyst preheating plays an important role in determining the extent of HC emission reduction for hybrid vehicles, especially immediately after the engine starts. This suggests that the role of the heated element of the EHC and its best size regime for hybrid vehicle application would be fundamentally different from those for conventional gasoline engine application. It should be mentioned that unlike conventional hybrid vehicles where energy for the EHC is obtained from on-board fuel, the energy for the EHC (and the battery) for PHEVs and EREVs could come entirely from nonfuel (and renewable) energy sources. In this paper, we first briefly summarize the major findings from previous studies on EHC application (postheating only) for cold-start emission control for conventional gasoline vehicles. We then analyze, using a mathematical model, the behavior of an EHC system (i.e., an inert electric heater followed by a three-way catalyst) under conditions likely to be encountered during hybrid vehicle operation. The performance characteristics of the EHC system are compared and contrasted with those for conventional EHC applications to illustrate the unique aspects of EHC application to hybrid vehicles. Topics of particular interest here include the effects of pre- and postheating on main converter thermal response and conversion performance, determination of optimum size regimes for the electric heater and light-off catalyst, and emission benefit of using multiple heaters (e.g., a small and a larger heater in series).

’ CONVENTIONAL EHC APPLICATIONS The purpose of this section is to provide a brief summary of the major findings from previous studies on conventional EHC applications involving only postheating of the electric heater. As mentioned above, it has been demonstrated both experimentally and theoretically that for cases with postheating only, decreasing the volume of an electric heater generally improves the emission performance of the EHC system during cold-start periods.4,7,9 This can be easily understood by examining how electric heater size variation affects the relative contributions of the electric heater (catalyzed metal-substrate monolith) and the downstream main converter (unheated ceramic-substrate three-way catalyst monolith) to the overall HC conversion performance of the EHC system. Figure 1 shows the results of such parametric

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Figure 1. Relative contributions of catalyzed electric heater (frontal area = 45.6 cm2) and unheated main converter toward HC emissions for a power level of 2.5 kW for 20 s duration. Also shown is the tailpipe HC emission predicted with the 0.4 cm long uncatalyzed electric heater (reproduced/replotted from Bissett and Oh10 with permission).

sensitivity calculations (for 20 s postheating at 2500 W) obtained using the transient one-dimensional monolith model described in our earlier paper.6 Shown in Figure 1 is the effect of electric heater volume on the predicted postheater and tailpipe HC emissions during cycle 1 of the Federal Test Procedure (first 125 s after cold start) for a GM vehicle equipped with a 3.8 L V6 gasoline engine. As expected, the postheater emission increases (i.e., decreased HC conversion over the heater) with decreasing heater volume as a result of the decreased residence time of gas within the heated element. It is interesting to note, however, that as the heater volume decreases, the HC conversion over the main converter (as given by the blue-shaded region in Figure 1) improves substantially, resulting in the lowest tailpipe HC emission levels in the regime of small heater volumes. Further analysis of the simulation results showed that the enhanced conversion performance of the main converter predicted with a small-volume heater is directly linked to its effectiveness at heating the main converter located downstream to the required reaction temperature.10 It is important to recognize in Figure 1 that there is a critical heater volume below which the emission benefit of decreasing electric heater volume disappears. Asymptotic analysis of the EHC model identifies this critical heater volume, Vh, which is given by the exhaust flow rate times its specific heat divided by the product of the interphase (gassolid) heat transfer coefficient within the heated element and its geometric surface area per unit volume.10 It was further shown that the recommended heater size (V) should be comparable in magnitude to Vh. When the electric heater is too large (V . Vh) the cold-start emission performance suffers from the slow heat-up rate of the heater caused by its large thermal mass. Also, decreasing the heater volume to values much smaller than Vh (i.e., V , Vh) is not desirable because it would lead to unnecessarily higher (potentially damaging) solid temperatures without producing additional emission reductions. In this smallest heater size regime, the 8445

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Industrial & Engineering Chemistry Research Scheme 1. Emission Control System Architecture Considered in This Work

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(i.e., washcoat) of uniform thickness and it is also assumed that the washcoat is sufficiently thin (typically 50 μm or less) so that its curvature is unimportant and diffusion resistance within the washcoat can be neglected.23 The governing model equations derived under these assumptions are given below: Solid-phase energy balance: ðψ0 þ fsb Fsb cp, sb þ fwc Fwc cp, wc Þ

emission performance of the EHC system becomes insensitive to heater size variation (e.g., for heater lengths shorter than 1 cm in Figure 1) because the higher solid temperatures within the smallest heater tend to be compensated for by the smaller interphase (solidgas) heat transfer area arising from the small heater volume. It is also interesting to note that a sufficiently small electric heater, such as the 0.4 cm-long heater shown in Figure 1, is predicted to be almost equally effective at reducing cold-start emissions regardless of whether or not the electric heater is catalyzed. This indicates that the primary function of a smallvolume electric heater is to transfer the supplied electrical energy downstream for rapid light-off of the main converter (i.e., heat transfer medium), rather than to achieve substantial conversion of HC emissions over itself (i.e., catalytic conversion device) and thus no significant emission benefit is expected by catalyzing the electric heater. However, the cold-start emission performance of the entire EHC system is enhanced significantly by increasing the noble metal loading in the upstream section of the main converter, which is the region activated by the heater-out gas at elevated temperatures.

’ MATHEMATICAL MODEL The emission control system architecture analyzed in this paper is shown in Scheme 1. The electric heater (EHC) and the three-way catalytic converter (TWC) are placed in series. The TWC has two bricks/catalysts, namely, the light-off (LO) catalyst followed by the main catalyst. It is assumed that the there is no heat loss (of the exhaust gas) in the section between the EHC and the TWC since they are very close to each other. In this work, we shall examine situations where the EHCTWC system is placed in the underfloor location of the exhaust system (i.e., far from the engine). A one-dimensional model is used for both the EHC and the TWC. Since the EHC and TWC are both of a monolithic structure, we first present a generic model for a monolith converter and then make appropriate simplifications relevant to the EHC (on a metallic substrate) and TWC (on a ceramic substrate). A detailed description of the model is given in the following section. Generic Model for the Monolith Converter. A singlechannel (i.e., channel-to-channel variations and interactions are ignored) one-dimensional model is used for describing the monolith converter.21,22 The axially varying gas phase temperature, velocity, and reactant concentrations are to be interpreted as cross-sectional averages within each channel. Axial diffusion of mass and heat is also assumed to be negligible in the gas phase. (Axial Peclet number, defined as the ratio of axial diffusion time to the axial convection (residence) time, is much greater than 1000 for both heat and mass transport, indicating dominance of the convective heat and mass transfer). The wall of the individual channel is coated with a porous, catalytically active layer

¼ fsb

∂Ts ∂t

X nrxn ∂ ∂Ts λsb aj ðzÞðΔHÞj Rj ∂z ∂z j¼1

þ hSðTg Ts Þ εσSL ðTs 4 Ta 4 Þ þ

PðtÞ AL

ð1Þ

Gas-phase energy balance: ∂Tg w cp, g ¼ hSðTs Tg Þ A ∂z

ð2Þ

Species mass balance in gas phase and catalyst surface (for i = 1, 3 3 3 , nsp): nrxn X w ∂xg, i ¼ km, i Sðxg, i xs, i Þ ¼ aj ðzÞRj si, j A ∂z j¼1

ð3Þ

Surface coverage equations: dθk ¼ Fk ðTs , xs , θÞ dt Fk ðTs , xs , θÞ ¼ 0

k ¼ 1, 3 3 3 , ndfq k ¼ ndfq þ 1, 3 3 3 , nsrf

ð4Þ

ð5Þ

Boundary conditions: xg, i ðz ¼ 0, tÞ ¼ xi, in ðtÞ Tg ðz ¼ 0, tÞ ¼ Tin ðtÞ

ð6Þ

∂Ts εσ ¼ ðTs4 Ta4 Þ @z ¼ 0 fsb ∂z ∂Ts εσ 4 ¼ ðTs Ta4 Þ @z ¼ L λsb fsb ∂z

ð7Þ

λsb

Initial conditions: Ts ðz, t ¼ 0Þ ¼ Tinit ðzÞ θk ðt ¼ 0Þ ¼ θk, init ðzÞ k ¼ 1, 3 3 3 , ndfq

ð8Þ

Here, Ts is the solid phase temperature, Tg is the gas temperature, xg and xs are the gas and solid phase mole fractions of species i. Here, nrxn represents the number of reactions and nsp represents the number of reacting species. The subscripts “wc” and “sb” represent 8446


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the washcoat and the substrate, respectively. The stoichiometric coefficients are given by si,j. In the above equations, θk are the coverage parameters and Fk is a function of the solid temperature, solid phase mole fractions, and the coverage parameters. Depending on the system under study, the coverage parameters are governed by differential equations or algebraic equations. Here, ndfq represents the number of surface coverage parameters determined by differential equations and nsrf represents the total number of surface coverage parameters. In the above equations, S and SL represent the geometric surface area per converter volume and external (skin) surface area of the monolith per converter volume, respectively. These, as well as all other variables, are defined in the nomenclature section. The solid-phase energy balance takes into account the axial heat conduction (term 1 in the right-hand side (rhs)), the heat generated by reactions (term 2), heat transfer between the gas and the solid (term 3), radiative heat loss from the external converter surface (term 4), and power input to the monolith (term 5). In the above equation, ψo, the parasitic heat capacity of the monolith heater9 is applicable only for the electrically heated system and this term usually takes into account the heat capacity of other elements (like the center pins, insulators, tabs, leads, etc.) which can contribute substantially to the total heat capacity of the monolith.10 The boundary conditions for the solid-phase energy balance given by eq 7 take into account the radiative heat loss from the inlet and outlet faces of the catalyst. Note that radiative heat losses are significant only for specific cases/situations, as will be discussed in detail later. The heat and mass transfer coefficients in the above model equations are determined by the following equations and correlations: Nu λg Dh Sh km, i ¼ ðcg Di, m Þ Dh h¼

ð9Þ

where the thermal conductivity of the gas (λg) and the mass diffusivity (Di,m) are given by24,25 λg ¼ 2:66 104 Tg 0:805 cg Di, m

rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi 1 1 3:85 105 Tg 0:75 þ Mi MN2 ¼ pffiffiffiffi pffiffiffiffiffiffiffi ð 3 ∑i þ 3 ∑N2 Þ2

ð10Þ

Here, Mi represents the molecular weight of the species in grams and ∑i represents the diffusion volume of the species (obtained from Table 11-1 in The Properties of Gases and Liquids26). We assume asymptotic (constant) Nusselt number (Nu) for simulation purposes. (Note that Nu = Sh). More details of the model and the simplifying assumptions can be found elsewhere.21,22 Model for the Electrically Heated Catalyst. The electrically heated catalyst is basically a metal-substrate monolith with or without catalyst coated on it. For the scope of this work, we shall consider only heaters which are not catalyzed (and not washcoated). Hence, the reaction term and the heat capacity of the washcoat can be dropped from eq 1 and the gas-phase species and surface coverage balance equations, eq 3, 4, and 5, need not be solved. The simplified one-dimensional model for the heater can now be written as follows.

Solid-phase energy balance: ðψ0 þ fsb Fsb cp, sb Þ

∂Ts ∂ ∂Ts λsb ¼ fsb þ hSðTg Ts Þ ∂z ∂t ∂z þ

PðtÞ εσSL ðTs 4 Ta 4 Þ AL

ð11Þ

The above equation is solved along with eq 2 and the corresponding initial and boundary conditions in eqs 6, 7, and 8. The channel cross-section is assumed to be close to a circular/sinusoidal geometry, and correspondingly a value of Nu = 4 is chosen.27 The above model can be further simplified for preheating (when there is no gas flow) and postheating (when there is gas flow). Preheating Model. When there is no gas flow in the heater, the molar flow rate w is zero and hence the solid and gas temperatures are equal from eq 2. Using this information and further assuming that the axial gradients in the temperature are small, the solid phase energy balance can be integrated from z = 0 to z = L and the simplified (and lumped) solid phase energy balance can be expressed as ∂Ts PðtÞ 2 εσ SL þ ðTs 4 Ta 4 Þ ð12Þ ¼ ðψ0 þ fsb Fsb cp, sb Þ AL L ∂t with the initial condition Ts ðt ¼ 0Þ ¼ Tinit

ð13Þ

The last term on the rhs of eq 16 represents the total radiative heat loss (from the external surface area and the catalyst faces) of the heated catalyst. The radiative heat loss term is important when the catalyst is heated through external power sources without exhaust gas flow, especially for small heaters. Postheating Model. When there is gas flow through the monolith channels, the heat transfer between the gas and the solid is significantly larger compared to the heat loss through radiation and hence the radiative heat loss terms in the solid phase energy balance and in the boundary conditions can be neglected. However, unlike the preheating case, significant axial temperature gradients are developed in the presence of gas flow through the monolith channels. The simplified solid phase energy balance equation can then be expressed as ∂Ts ∂ ∂Ts PðtÞ λsb ¼ fsb þ hSðTg Ts Þ þ ðψ0 þ fsb Fsb cp, sb Þ ∂z AL ∂t ∂z ð14Þ with the boundary conditions λsb

∂Ts ¼0 ∂z

@z ¼ 0, L

ð15Þ

and the gas phase energy balance is given by eq 2. Model for the Three-Way Catalyst. For the scope of this work, we shall consider only a conventional ceramic-substrate based TWC which is not heated by external power sources. Hence, the parasitic heat capacity ψo and the external power input term can be dropped from the solid phase energy balance. Since there are no physical/chemical processes occurring in the TWC during preheating, there is no need to solve the TWC balance equations during the preheating period. During postheating the heat generated by the reactions and the heat transfer between the gas and the solid are significant as compared to the radiative heat loss and hence the radiation terms can be dropped. This 8447



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Table 1. Main Reactions in TWC reactions

heat of reaction (J/mol)

oxidation reactions 1 CO þ 0.5O2 f CO2 2 C3H6 þ 4.5O2 f 3CO2 þ 3H2O 3 C3H8 þ 5O2 f 3CO2 þ 4H2O 4 H2 þ 0.5O2 f H2O NO reduction reactions 5 CO þ NO f CO2 þ 0.5 N2 6 C3H6 þ 9NO f 3CO2 þ 3H2O þ 4.5N2 7 H2 þ NO f H2O þ 0.5N2 water-gas and steam reforming reactions 8 CO þ H2O f CO2 þ H2 9 C3H6 þ 3H2O f 3CO þ 6H2 ceria reactions (oxygen storage) 10 2Ce2O3 þ O2 f 4CeO2 11 Ce2O3 þ NO f 2CeO2 þ 0.5N2 12 CO þ 2CeO2 f Ce2O3 þ CO2 13 C3H6 þ 12CeO2 f 6Ce2O3 þ 3CO þ 3H2O 14 C3H8 þ 14CeO2 f 7Ce2O3 þ 3CO þ 4H2O 15 H2 þ 2CeO2 f Ce2O3 þ H2O

2.83 105 1.93 106 2.04 106 2.42 105

’ SYSTEM DESIGN AND OPERATING PARAMETERS For simulation purposes in this study, we construct a set of engine-out (heater inlet) exhaust data so as to represent typical engine-out conditions during a cold-start period. The exhaust gas temperature is assumed to rise exponentially from an initial value (To = 22 C) to a maximum asymptotic value (Tmax = 325 C) with a time constant τ (=17 s) according to the following expression and is shown in Figure 2.

3.73 105 2.74 106 3.32 105 4.12 104 þ3.74 105 2 105 1.90 105 1.83 105 4.77 105 4.95 105 1.42 105

T ¼ To þ ðTmax To Þð1 et=τ Þ

simplifies the solid phase energy balance to ðfsb Fsb cp, sb þ fwc Fwc cp, wc Þ

∂Ts ∂t

X nrxn ∂ ∂Ts λsb ¼ fsb aj ðzÞðΔHÞj Rj þ hSðTg Ts Þ ∂z ∂z j¼1 ð16Þ with the boundary conditions λsb

∂Ts ¼0 ∂z

@z ¼ 0, L

ð17Þ

The Nusselt number for gassolid heat transfer is assumed to be a constant. The channel cross-section is assumed to be close to a square geometry and correspondingly a value of Nu = 3 is chosen.27 For the reactions occurring in the TWC, the species (eq 3) and surface coverage equations (eqs 4 and 5) need to be solved along with the above-mentioned solid phase energy balance and the gas phase energy balance (eq 2). The set of reactions occurring in the TWC are listed in Table 1 along with the respective heat of reactions. LangmuirHinshelwood-type rate expressions are used for the various reactions22 with rate constants and kinetic parameters obtained from Ramanathan and Sharma.28 For this set of reactions, the number of surface coverage parameters is 1; that is, nsrf = 1 (with ndfq = 1) and the coverage (or the oxidation) parameter θ is the fraction of the oxygen storage component (e.g., cerium) in the higher oxidation state, as given by θ¼

mol/mol-site/sec, where mol-site refers to the active sites which actually participate in the catalytic reactions. By default, we take all the cerium in the TWC to be initially in the oxidized state (more stable) so that θ = 1 at t = 0.

moles of CeO2 moles of CeO2 þ 2 moles of Ce2 O3

The fraction of sites in the lower oxidation state required in reactions 10 and 11 is (1 θ). The equation for the surface coverage can be written as dθ ¼ FðTs , cs , θÞ dt ¼ ð4R10 þ 2R11 Þ ð2R12 þ 12R13 þ 14R14 þ 2R15 Þ In the above equation, the Rj variables represent the specific reaction rate or the turnover rate for the jth reaction and is expressed in

ð18Þ

The mass flow rate is assumed to be constant at 19 g/s and the exhaust species concentrations are approximated by a piecewise constant function: constant for the first 4 s and smaller constants for t > 4 s. The higher concentration values for CO, H2, and HCs during the first 4 s are associated with fuel-rich engine operation immediately following engine cold start at the beginning of the FTP driving cycle. The species concentrations listed in Table 2 represent averaged measured values from actual FTP tests, and were used to construct the synthetic engine-out concentrations for simulations. Table 3 gives the standard set of catalyst and geometric parameters for the EHC and TWC. Unless noted otherwise, the standard parameters are used for all simulations. A standard preheating time of 150 s is chosen in this study since vehicle tests show that a PHEV or EREV can be driven entirely on battery during the first 150 s of the FTP drive cycle. The engine operation is generally required only when the vehicle needs more power or the battery needs charging. In this study, the metal substrate temperature is not allowed to exceed 800 C in view of its thermal stability. For a preheating time of 150 s and a heater of length L (=0.9525 cm), the maximum power that can be supplied without exceeding the substrate temperature of 800 C is 2 kW. The corresponding power levels for heater lengths of 2L and 3L are 2.5 and 3.2 kW, respectively. It should be mentioned that at a given power input, the heater lengths 1L, 2L, and 3L will have different temperatures (TL > T2L > T3L) at the end of preheating because the power input refers to the total power distributed over the entire heater volume and not power per unit heater volume. It may also be worth noting that during the preheating time of 150 s the temperature of the heater does not necessarily reach steady-state for some heater lengths.

’ OPTIMAL HEATER SIZE AND SYSTEM BEHAVIOR WITH PREHEATING As discussed in the previous section, for conventional applications involving postheating only (i.e., electrical heating only when the engine is on), small electric heaters have been shown to be optimal because they tend to store little energy and almost all of the supplied electrical energy is used to quickly raise the temperature of the gas passing through the EHC, thus efficiently heating the main converter located downstream for rapid catalyst light-off.10 However, in hybrid vehicles, the advantage of preheating during extended engine-off periods (no exhaust flow) can be utilized for enhanced emission reduction. Preheating serves to store heat in the EHC which can then be transferred to the exhaust gas once the engine is turned on. Since converter preheating is a unique aspect of EHC application 8448



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Figure 2. Constructed engine-out exhaust gas temperature as a function of time.

Table 2. Species Concentrations Used to Construct Synthetic Engine-out Composition

Table 3. Catalyst and Geometric Properties of the EHCTWC System

concentration (in ppm) exhaust species

t e 4 sec

t > 4 sec

CO O2 NO C3H6 C3H8 CO2 H2 H2O

17668 61196 389 545 136 110000 5169 112923

4000 10000 200 320 80 110000 1082 104368

to hybrid vehicles, it is instructive to examine the impact of preheating on the thermal and emission performance characteristics of the EHCTWC system. The results of such an analysis provides understanding of the roles of the individual components of the EHCTWC system (i.e., heater, LO, and main catalysts) and the nature of the interactions between them during and after preheating. For the case of preheating only, a small heater reaches higher temperatures during preheating compared to a large heater. This higher temperature of the small heater leads to rapid transfer of the heat to the gas when the flow is turned on (due to the high ΔT (=Ts Tg) between the gas and the heater). Because of its low thermal mass (and the rapid heat transfer to the gas), however, small heaters will also cool off rapidly and thus can stop transferring heat to the gas even before the engine exhaust reaches typical catalyst light-off temperatures (e.g., 250 C at 25 s; in Figure 2). On the other hand, a large heater would transfer the stored heat to the gas more slowly over a longer time period. Such differences in heat transfer behavior between small and large heaters are illustrated in Figure 3. Figure 3 shows the time variation of the heater-out gas temperature for three different heater sizes (1L, 2L, and 3L) after preheating for 150 s at three different power levels (0.5, 1.2, and 2 kW). As can be seen, at the end of preheating (t = 0 s) the small heater has the highest temperature and the large heater has the lowest temperature. Once the engine is turned on (i.e., in the presence of gas flow), the gas gets

TWC

length (L in cm) frontal area (A in cm2) converter volume (in liters) washcoat volume fraction (fwc) substrate volume fraction (fsb) hydraulic diameter (Dh in mm) catalyst loading (in g/L) parasitic heat capacity (ψo in J/(m3 3 K))

EHC

LO catalyst

main catalyst

0.9525 115 0.1095 0 0.06 1 0 1.04 106

7.493 110 0.8242 0.2 0.1444 0.6856 7.27 (Pd) 0

26.543 110 2.9198 0.2 0.1646 1.0123 3 (Pd/Rh = 5:1) 0

heated up by the heater at a rate proportional to the ΔT (=Ts Tg) between the heater and the gas. For small heaters, this difference is largest (because of the higher heater temperature at the end of preheating) during the first few seconds and the gas gets heated to higher temperatures quickly. This is seen by the blue curves having higher temperatures in the first few seconds. However, because of their small heat capacity and rapid heat transfer to the gas, small heaters will also cool quickly, and with the resulting smaller ΔT the gas does not get heated up very effectively at later times (as seen by the quick drop in the gas temperature). After a while the driving force for heat transfer changes sign (because of the decrease in heater temperature and increase in engine-out gas temperature), leading to heat loss from the gas to the heater and as a result, the heater-out gas temperature goes through a minimum. Thus, small heaters provide higher heater-out gas temperatures in the first few seconds, with a temperature minimum occurring at early times. On the other hand, the gas temperature from large heaters is not very high immediately after the engine is turned on, but decreases rather slowly with a temperature minimum reached at a later time. As will be shown later, the heater-out gas temperature following preheating plays an important role in determining the performance of the LO catalyst. To have the best performance of the LO catalyst, the catalyst should be quickly heated to its 8449



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Figure 3. Heater-out gas temperatures as a function of time for three different heater sizes following 150 s of preheating at three different power levels.

light-off temperature (around 270 C) and sustain the reaction, which requires that the heater-out gas temperature stay higher than the catalyst light-off temperature for a sufficiently long time. At lower power levels (e.g., 0.5 kW), the heater-out gas temperatures are too low for the large and midsized heaters (lower than light-off temperatures) and the small-sized heater has a significantly higher heater-out gas temperature (including higher minimum temperature) at early times and hence would be expected to give a better light-off performance from the LO catalyst. At higher power levels (e.g., 2 kW), the heater-out gas temperatures are significantly higher than the light-off temperature for all heater sizes and the large-sized heater has significantly higher heater-out gas temperature (including higher minimum temperature) for most times and hence would be expected to give better light-off performance of the LO catalyst. At intermediate power levels (e.g., 1.2 kW), the midsized heater has the best emission performance as it maintains the highest overall temperatures during the first 30 s and hence is expected to give a better light-off performance in the LO catalyst. Figure 4 shows the cumulative HC emissions (in mg/mi, over 100 s after the preheating period) out of the LO catalyst as a function of preheating power levels (150 s preheating time without postheating in all cases) for three different EHC sizes (variable lengths of 1L, 2L, and 3L with 115 cm2 frontal area) and at two different LO catalyst loading levels. It can be seen that for each case, there is a critical power level below which no significant emission reduction is predicted. This can be attributed to the dominance of the cooling effect of the incoming engine exhaust over the preheating, resulting in the quenching of the reaction in the LO catalyst. Figure 4 also shows that at the both LO catalyst loadings considered, a small heater gives the lowest cumulative emissions at lower preheating power levels. This is because at lower power levels only small-volume heaters can be heated to temperatures higher than a typical catalyst light-off temperature at the end of the preheating period, thereby transferring the stored energy to the exhaust gas quickly and efficiently after the

engine is turned on. However, at higher preheating power levels, we observe a larger heater size to be optimal. For example, at preheating power levels higher than 1300 W, a heater with length 2L gives slightly better emission performance with the LO catalyst having the base loading. As the LO catalyst loading is reduced (either by design or as a result of catalyst aging), the demarcation between the heater sizes becomes more pronounced at power levels higher than 1000 W. It should be noted that the dependence of the optimum heater size on preheating power level shown in Figure 4 is qualitatively consistent with the heater size effect on the heater-out gas temperature profiles at various power levels discussed above (and shown in Figure 3). Figures 3 and 4 clearly show that there exists an optimum heater size for preheating only and the optimum heater size depends on the preheating power level. This is in contrast to cases involving postheating only, where a small-volume heater has been shown to be optimal regardless of the power level.10 Given this interesting observation, it would be instructive to examine in detail the thermal response and emission performance characteristics of the EHC TWC system following preheating of the electric heater. For analyzing the various processes occurring in the EHC TWC system during typical operation with preheating only, we choose the base case to have the following specifications: heater length of 1L with 150 s of preheating at 0.9 kW (no postheating) and all other system parameters as given in Table 3. Figure 5 shows the engine-out (heater inlet) gas temperature and the gas temperature out of the heater (LO catalyst inlet) for the base case. The heater bed temperature at the end of the preheating was 583 C. As can be observed from the Figure, the heater-out gas temperature starts at a high value (575 C) thanks to the energy stored in the heater during the preheating period, but drops rapidly to a minimum temperature of 193 C (reached at ∼15 s) and then starts to increase slowly. Also, it is important to observe that the heater-out gas temperature is higher than the engine-out gas temperature for the first 14 s during which the heat transfer is from the heater to the gas, but there is a period of 8450



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Figure 4. Effect of preheating power level (150 s heating time) on cumulative HC emissions out of the LO catalyst (at 100 s) for three different EHC lengths and two different LO catalyst loadings.

Figure 5. Engine-out and EHC-out gas temperature as a function of time (following 150 s of preheating at 0.9 kW).

time (1460 s) during which the heater-out gas temperature is lower than the engine-out gas temperature, giving rise to heat transfer from the gas to the heater. This process of reverse heat transfer from gas to heater is not desirable and as will be shown later, it can be avoided by postheating for a sufficiently long period of time. The rapid drop in the heater-out gas temperature at early times shown in Figure 5 gives rise to an interesting thermal response of the LO catalyst after the engine is turned on. The low inlet gas

temperature cools down the upstream section of the LO catalyst (with the reaction quenched there) and since the heat generated by the initial light-off is not enough to sustain the reactions in the presence of the exhaust flow, the reaction zone is pushed downstream and eventually moved out of the LO catalyst. This behavior is illustrated in Figure 6, which shows the time variation of the solid temperature profile along the length of the LO catalyst for the base case. The catalyst bed temperature starts from 25 C and the hot gas out of the EHC heats up the LO 8451



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Figure 6. Time variation of bed temperature profiles along the length of the LO catalyst following 150 s of preheating for the base case.

Figure 7. Gas-phase concentrations (in mole-fractions) and temperature out of the LO catalyst as a function of time following 150 s of preheating for the base case. The arrow indicates the rise in species concentrations and the corresponding drop in temperature.

catalyst, leading to catalyst light-off near the catalyst inlet and the resulting reaction exotherm (as evidenced by the temperature peaks at 10 and 20 s). However, the cool incoming engine exhaust cools down the inlet section of the LO catalyst and pushes the light-off (or reaction) front downstream by convection during the first 30 s. Eventually, by 40 s into the process, the reaction front moves out of the LO catalyst, resulting in the quenching of the reaction in the face of the cooling effect of the incoming engine exhaust gas. This cooling of the LO catalyst by

the engine exhaust leads to significant emission breakthroughs between 30 and 60 s, as illustrated in Figure 7. During the 4060 s period, however, the engine exhaust gas approaches catalyst light-off temperatures (see Figure 5), and consequently, the upstream section of the LO catalyst starts to be heated up and lit-off again (see Figure 7), but this process is rather slow and not as dramatic as the initial rise in bed temperature. Figure 7 shows the corresponding gas-phase concentrations and temperature out of the LO catalyst for the base case. The first 8452



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Figure 8. Cumulative emissions of CO and HC out of the LO and main catalysts for the base case following 150 s of preheating.

light-off is seen clearly by the rapid drop in the concentrations of the exit gas during the first 23 s. After the first light-off, the concentrations remain at almost zero (except for C3H8, slowoxidizing hydrocarbon, which does not achieve 100% conversion). The gas temperature also rises during this period as a result of reaction exotherm. However, because of the cooling of the inlet gas, the exotherm generated during the first light-off is not able to sustain itself, leading to the corresponding increases in the LO catalyst exit concentrations (except for NO) starting at around 30 s. Once the catalyst inlet gas temperature rises beyond the required reaction temperature (>40 s into the process; see Figure 5), the second light-off occurs and the exit gas concentrations drop again. Note that, except for C3H8 all other concentrations drop to zero during the second light-off. Because of the nonzero exit concentration of C3H8, the cumulative HC (sum of the slow-oxidizing hydrocarbon C3H8 and fast-oxidizing hydrocarbon C3H6) emissions out of the LO catalyst will not be flat after the second light-off (see Figure 8 below). Note that achieving 100% conversion for the slow-oxidizing HC requires significantly higher catalyst inlet gas temperatures and/or higher catalyst loadings. The cumulative emissions of CO and HC out of the LO catalyst (solid lines) and the main catalyst (dotted lines) for the base case are shown in Figure 8. It is easily seen that there are changes in the slope of the cumulative emission profile from the LO catalyst. During the first two seconds, there is a steep rise in cumulative emissions since the LO catalyst remains inactive (still below the required reaction temperature). Between 2 and 30 s, the LO catalyst is lit off and there is no increase in cumulative emissions for CO and slow increase in cumulative emissions for HC (due to the slow-oxidizing HC). However, as explained earlier, starting at around 30 s, both the CO and HC emission breakthroughs increase rapidly (as evidenced by the increase in slope) and this can be attributed to the cooling of the LO catalyst after the first light-off. At around 60 s, there is a second catalyst

light-off during which CO emissions do not increase again and the HC emission increases slowly. This sequence of events is referred in the figure as “Initial light-off”, “Emission breakthroughs due to LO catalyst cooling” and “Second light-off”. It is interesting to note that the main catalyst (at least at the relatively high noble metal loading considered) tends to mitigate the effect of the LO catalyst cooling and the resulting emission breakthroughs from the LO catalyst. The prediction of no significant emission breakthroughs from the main catalyst can be attributed to the fact that the outlet gas from the LO catalyst heats up the main catalyst during the first 30 s (when there are no emission breakthroughs from the LO catalyst) as the reaction front moves downstream. By the time the emission breakthroughs from the LO catalyst reach the main catalyst (between 30 and 60 s), the upstream section of the main catalyst has been heated to high enough temperatures to achieve significant conversion to eliminate them. This behavior is illustrated in Figure 9 which shows the time variation of the solid temperature profile along the length of the main catalyst following 150 s of preheating. It should be mentioned that the ability of the main catalyst to eliminate/reduce the emission breakthroughs from the LO catalyst depends significantly on the active metal loading of the main catalyst. Figure 10 shows how the cumulative CO and HC emissions out of the main catalyst are affected by the active metal loading of the main catalyst. Also shown in the figure is the cumulative emissions leaving the LO catalyst for comparison purposes. As can be observed, for lower levels of catalyst loading, the emission breakthroughs from the LO catalyst are not completely eliminated, contributing significantly to the tailpipe emissions from the EHCTWC system.

’ EFFECTS OF POSTHEATING Our analysis in the previous section clearly shows that the base-case EHCTWC system with preheating only (150 s at 0.9 kW power) is not effective at sustaining the early light-off in the 8453



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Figure 9. Time variation of bed temperature profiles along the length of the main catalyst for the base case following 150 s of preheating.

Figure 10. Cumulative emissions of CO and HC out of the main catalyst with various active metal loadings. Post-LO catalyst emissions are also shown for comparison purposes.

LO catalyst in the face of the cooling effect of the incoming exhaust gas. Though the main catalyst gets heated up during the initial light-off of the LO catalyst (refer to Figure 9), the extent of emission reduction over the main catalyst can be rather small when the active metal loading in the main catalyst is low, as illustrated in Figure 10. Thus, it is important to understand the thermal interaction between the EHC and the LO catalyst, with a goal to optimize the electrical heating strategy to make sure that the gas leaving the EHC stays hot enough to have rapid and

sustainable light-off in the LO catalyst in the face of the cooling effect of the exhaust gas. In this section, we examine how the thermal response and emission performance characteristics of the LO catalyst are affected by the various heating strategies and discuss the effectiveness of various heating strategies in improving the entire EHC system performance. A focus shall be given to minimizing the emission breakthroughs from the LO catalyst after the initial light-off. In addition to improved heating strategies, the initial light-off can also be made to sustain itself by 8454



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Figure 11. Heater-out (or LO catalyst inlet) gas temperature at various preheating power levels (150 s preheating and no postheating in all cases).

changing the LO catalyst’s design parameters such as increasing the noble metal loading and the size of the LO catalyst. Later in the paper, we will briefly discuss two cases: (1) effect of increasing the size of the LO catalyst for fixed noble metal content and (2) effect of varying the noble metal content in the LO catalyst of fixed volume. First, we consider increasing the preheating power level (still with no postheating) and its impact on the emission breakthroughs from the LO catalyst. As mentioned earlier, there is a practical constraint of restricting the heater temperature to a maximum of 800 C (because of thermal stability issues of the metal substrate) and hence the preheating power level is restricted to a maximum of 2 kW for the base heater length of 1L. Figure 11 shows the gas temperature out of the heater at various preheating power levels. As can be observed from the figure, the heater-out gas temperature changes significantly only during the first 1015 s after which the curves closely follow the base case (0.9 kW) and there is only a slight improvement (increase) in the lowest temperature reached by the gas coming out of the heater; for 2 kW preheating, the lowest temperature reached is 204 C as compared to 193 C for the base case. However, the initial high temperatures (predicted for the first 1015 s at higher preheating power levels) can affect the performance of the LO catalyst significantly. In Figure 12, we show the cumulative HC emissions out of the LO catalyst at 100 s for various preheating power levels. As can be observed from the figure, the first light-off does not sustain itself even at 2 kW preheating power, exhibiting emission breakthroughs at t > 35 s. However, at higher power levels, the HC conversion is 100% (represented by flat profile in the cumulative HC emissions) for both fast and slow-oxidizing HCs during the initial light-off (first 30 s) but the emission performance characteristics at t > 35 s are similar (indicated by the slope of the cumulative emissions profile) at all preheating power levels, consistent with the essentially identical catalystinlet gas temperature profiles shown in Figure 11. It is interesting to note that, as expected, lower emissions are predicted at higher preheating

power levels, but there seems to be a diminishing return when increasing the power level beyond 1.5 kW. In addition, preheating can only increase the catalyst inlet gas temperature during a short period of time following the engine start, thus providing no significant emission benefit at later times (e.g., after the initial catalyst light-off). This suggests that the addition of postheating might be an effective way to improve the emission performance of the LO catalyst. Figure 13 shows the beneficial effect of increasing postheating time on the gas temperature out of the heater (LO catalyst inlet) following 150 s of preheating at 0.9 kW. As can be observed from the figure, the EHC-out gas temperature at early times (first few seconds) is determined primarily by the energy stored in the heater during the preheating period, with little contribution from postheating. However, at later times, there is a clear advantage of the case with both pre- and postheating over the base case with preheating only. As the postheating time increases from 0 to 15 s, the minimum outlet gas temperature increases from 190 to 221 C. However, as we further increase the postheat time, we start to see two minima in the EHC outlet gas temperature: one minimum during postheating and the other after postheating is switched off. For 20 s or longer postheating, the first minimum (233 C) occurs at around 15 s and the second minimum (>233 C) occurs a few seconds after the postheating is switched off. Figure 14 shows the corresponding CO and HC cumulative emissions for various postheating times (at 0.9 kW power). As the postheating time increases, the EHC outlet gas temperatures increase and this leads to lower emission breakthroughs of CO and HC emissions leaving the LO catalyst between 30 and 50 s, compared to the base case (no postheating). However, the emission breakthroughs after the initial light-off are not completely eliminated even with 50 s postheating, indicating that the LO catalyst still cools off (with the reaction quenched) during and after the initial light-off. Since the addition of postheating at the relatively low power level of 0.9 kW to the base case (150 s preheating at 0.9 kW) did 8455



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Figure 12. Cumulative HC emissions from the LO catalyst at various preheating power levels (150 s preheating and no postheating in all cases).

Figure 13. Heater-out (or LO catalyst inlet) gas temperature for various postheating times (0.9 kW postheating power and 150 s preheating at 0.9 kW). Also shown is the engine-out gas temperature.

not eliminate the emission breakthroughs even with 50 s postheating time, we now consider some other heating strategies to improve the performance of the EHCTWC system. Specific cases of our interest include (1) increasing postheating power level and (2) increasing both postheating power level and heating time. We shall discuss each of those cases in the following subsections and for comparison purposes we now redefine the base case to have the following specifications: heater length, 1L with 150 s of preheating at 0.9 kW and 10 s of postheating at 0.9 kW; all other parameters given in Table 3.

(1). Effect of Increasing Postheating Power Level (with 10 s Heating Time). Increasing the postheating power level is

not expected to be a promising strategy, at least with the 10 s postheating time considered here, because of the large parasitic heat capacity of the heater and the large cooling effect of the incoming exhaust gas at early times. Figure 15 shows the heaterout gas temperature at various postheating power levels (postheating time fixed at 10 s, and 150 s preheating at 0.9 kW). As can be observed from the figure, there is not a significant change in the heater-out (or catalyst inlet) gas temperature, except for a rather 8456



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Figure 14. Cumulative HC and CO emissions from the LO catalyst for various postheating times (0.9 kW postheating power and 150 s preheating at 0.9 kW).

Figure 15. Heater-out (or LO catalyst inlet) gas temperature at various postheating power levels (10 s postheating time and 150 s preheating at 0.9 kW).

small increase in the minimum gas temperature reached between 15 and 20 s. Simulations confirm that this small increase in temperature leads to only moderate improvement in the LO catalyst performance—cumulative HC emissions out of the LO catalyst at 100 s for postheating power levels of 0.9 and 2 kW are 16.3 and 14.2 mg/mi, respectively—at least for the relatively large electric heater (109.5 cc) and the short postheating time (10 s) considered here. Thus, we are led to the conclusion that 10 s postheating, even at 2 kW power, can provide only limited emission benefit and that longer heating times may be more effective at sustaining the initial light-off (see Figures 13 and 14).

(2). Effect of Increasing Postheating Power Level and Heating Time. Given the limited emission benefit of postheating

of a short duration (e.g., 10 s) discussed above, it would be instructive to examine the effect of postheating power levels at a relatively long postheating time of 40 s. Figure 16 shows the EHC-out gas temperatures for a postheating time of 40 s at various postheating power levels (still with 150 s preheating at 0.9 kW). At the highest postheating power level of 2 kW and 40 s of postheating time, the minimum temperature reached by the EHC-out gas is now around 282 C. This compares with the minimum temperature of 233 C predicted at 0.9 kW and 40 s of postheating time (see Figure 13). This clearly shows that an increase in both postheating power level and heating 8457



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Figure 16. Heater-out (or LO catalyst inlet) gas temperature at various postheating power levels (40 s postheating and 150 s preheating at 0.9 kW).

Figure 17. Cumulative HC emissions from the LO catalyst for various postheating power/time combinations (150 s preheating at 0.9 kW in all cases).

time is necessary for significant reduction of the emission breakthroughs from the LO catalyst. In Figure 17, we summarize cumulative HC emissions predicted for various combinations of postheating power and postheating time. It is easily seen from the figure that postheating at 2 kW for 30 s or longer provides high HC conversion during the first light-off (2 to 30 s), but postheating time of 40 or 50 s is required to have small emission breakthroughs after the initial light-off and significantly lower cumulative emissions at 100 s. At power levels lower than 2 kW (even with 40 s postheating time), the emission breakthroughs are significant and cumulative emissions at 100 s are higher. For CO (not

shown here), the model predicted no emission breakthroughs at power levels greater than 1.5 kW (with postheating time of 40 s or more). Hence, it can be concluded that a higher power level (>1.5 kW) with postheating time of 40 s or longer is required to have significantly lower emission breakthroughs.

’ SMALL HEATER VOLUME FOR POSTHEATING AND A LARGE HEATER FOR PREHEATING From the analysis of the various heating strategies considered in the previous section, it is clear that for the relatively large 8458


Industrial & Engineering Chemistry Research Scheme 2. Two-Heater Configuration for Exhaust Emission Control

heaters considered, postheating at higher power levels (>1.5 kW) for longer durations (>40 s) is required for effective emission reduction by the LO catalyst. Also, the results of Figure 4 show that for cases with preheating only, relatively large heater volumes are required to achieve sufficient emission reduction to meet future emission regulations (e.g., SULEV standards), especially after significant catalyst aging. However, the large thermal mass (or heat capacity) of the heater leads to inefficient operation of the heater during postheating, and this shortcoming can be overcome by having another heater of smaller size (placed in series between the existing heater and the LO catalyst), which due to its low thermal mass, will be heated up rapidly to a higher temperature and hence can heat up the gas much faster during postheating.10 With the addition of the smaller heater, the larger heater can be used primarily for storing heat during preheating while the smaller heater can be used to quickly heat the gas during postheating. There can also be other configurations like having a small heater in parallel to the existing larger heater. In this configuration, for a few seconds after the engine is turned on, the gas is allowed to flow through the existing (larger) heater without any postheating so that the heat stored during preheating can be transferred to the gas (and during this time the small heater can be preheated, if necessary) and before the flow is directed entirely through the postheated small heater. However, in this work we shall limit our analysis to cases with a small heater placed in series with the existing heater (as this configuration is more practical in terms of space constraints and would not require a diverter valve, etc.), as shown in Scheme 2. The specifications of the small heater (SH) are identical to the large heater (LH) except for the shorter heater length and the smaller parasitic heat capacity. The length of the small heater is assumed to be L/3 and the parasitic heat capacity ψo is assumed to be 0.24 106 J/(m3 3 K). In this new configuration, both the SH and the LH are heated during preheating and a plot of the heater temperature during preheating (for 150 s at 0.9 kW) is shown in Figure 18. It can be seen that the temperature of the large heater increases slowly during preheating, reaching 583 C at the end of the preheating period. However, the small heater is heated up much more quickly (e.g., to 583 C in 17 s) and reaches a steady state temperature of 630 C in 34 s as the radiative heat loss is balanced with the electrical heat supply. This shows that we do not have to preheat the small heater for 150 s and the preheating time could be reduced to 34 s (i.e., preheating can be switched on 34 s before expected engine turn-on). In general, if the heater reaches a steady-state temperature during preheating, the preheating time could be reduced to a value equivalent to the time taken to reach the steady state, thereby reducing the energy consumption. To compare the system with SH to a system without SH, it is important to maintain the same total energy consumed during

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preheating and postheating. The total energy consumption during preheating for systems without SH is 0.0375 kWh (900 W for 150 s). For systems with LH and SH, this energy needs to be distributed between the LH and SH. It is observed from Figure 18 that the SH reaches high temperatures within 17 s. Hence, one of the possible ways (such that both LH and SH have high temperatures at the end of preheating) to distribute the preheating energy (900 W for 150 s) will be to preheat the LH at 0.9 kW for 133 s (0133 s) and preheat the SH at 0.9 kW for the last 17 s (from 116 to 133 s). For the systems with LH only, the LH is preheated at 0.9 kW for 150 s. Unless mentioned otherwise, the above-mentioned preheating strategy is used for simulations of systems with SH in this section. In Figure 19, we compare the LO catalyst inlet temperatures for the cases with and without an additional small heater. This is shown for two different postheating times (40 and 100 s). When the small heater is present, the preheating is done at 0.9 kW for 133 s (0133 s) for the LH and 17 s (116133 s) for the SH followed by postheating of the SH only at 0.9 kW for the specified postheating time. For the case without the SH (i.e., LH only), the preheating is done at 0.9 kW for 150 s for the LH followed by postheating of the LH at 0.9 kW for the specified postheating time. It is observed that the catalyst inlet gas temperatures during the postheating are generally higher for the case with the SH compared to the case without the SH. However, once the postheating is switched off, the gas temperature coming out of the SH becomes lower (and cools off rapidly) than the case with LH only (no SH). This can be attributed to the fact that in the presence of gas flow, heat is transferred from the small heater to the gas quickly (because of its lower thermal mass and higher ΔT), decreasing the temperature of the SH rapidly. Thus, the SH is more effective at heating the gas during postheating but it loses heat rapidly once the power is turned off. This supports the notion that the additional SH will provide a significant improvement when the performance of the system is dominated by postheating (i.e., when postheating power/time is large or when preheating power/time is small). In Figure 20 we compare the cumulative emissions out of the LO catalyst for the cases with (“SH”) and without (“LH”) a small heater at postheating power levels of 0.9 kW and 2 kW for a postheating time of 100 s. The blue curves represent the cases with the small heater (in addition to LH) and the red curves represent the cases without the small heater (i.e., LH only). It is found that for the SH case, the initial light-off occurs slightly earlier (for both HC and CO) followed by higher HC conversion (close to 100% for 2 kW postheating power) during the 35 s or so. However, there is an increase in slope (for both SH and LH cases) after the first light-off which is indicative of emission breakthroughs from the LO catalyst because of the cooling by the incoming exhaust. At both power levels of 0.9 and 2 kW, the cumulative emissions for the SH case are lower than for the LH case throughout the entire period considered.

’ ENTIRE SYSTEM (EHCTWC) PERFORMANCE AND OPTIMAL HEATER AND LO CATALYST VOLUME FOR COMBINED PRE- AND POSTHEATING In the last two sections, we discussed how the thermal response and emission performance characteristics of the LO catalyst are affected by various catalyst heating strategies (preand postheating) and changes in the design parameters and configuration of the heater, including the heater size and addition 8459



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Figure 18. Temperature rises of the small and large heaters during preheating (for 150 s at 0.9 kW).

Figure 19. Heater-out (or LO catalyst inlet) gas temperature with and without the additional small heater (SH) for two different postheating times (at 0.9 kW postheating power).

of a small heater . In this section, we look into the performance of the entire EHCTWC system (i.e., including the main catalyst). It was shown earlier (refer to Figures 810) that the main catalyst may or may not be able to eliminate the emission breakthroughs from the LO catalyst, depending on the active metal loading level in the main catalyst. We observed that the outlet gas from the LO catalyst heats up the main catalyst in the first 30 s (when there are no emission breakthroughs from the LO catalyst) as the reaction front moves downstream and by the time the emission breakthroughs from the LO catalyst reach

the main catalyst (between 30 and 60 s), the upstream section of the main catalyst is hot enough (see Figure 9) to provide significant conversion, resulting in lower tailpipe emission levels. (refer to Figure 10). Since a larger catalyst volume can prevent the reaction front from moving out of the LO catalyst prematurely, it can be expected that a longer LO catalyst (i.e., larger volume with fixed frontal area) may improve the emission performance. However, for a fixed amount of active metal in the LO catalyst, a larger catalyst would mean lower active metal concentration per unit 8460



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Figure 20. Cumulative CO and HC emissions from the LO catalyst at two different postheating power levels for a postheating time of 100 s.

Figure 21. Cumulative HC emissions out of the LO catalyst for various different LO catalyst volumes for fixed total active metal content. Heating strategy: 150 s of preheating at 0.9 kW followed by 10 s of postheating at 0.9 kW.

catalyst volume and hence the light-off performance could be compromised, as illustrated in Figure 21. As the LO catalyst volume is increased from its base value (0.8242 L) to 3.297 L, the initial light-off becomes sluggish. However, at intermediate volumes (1.6482.473 L), the emission breakthroughs are lower overall, resulting in the lowest cumulative emissions out of the LO catalyst at 100 s. The importance of LO catalyst volume in the design of EHCTWC systems is discussed in more detail in Figure 22 below.

Figure 22 shows the relative contributions of the LO catalyst and the main catalyst to the cumulative HC and CO emissions (at 100 s) as a function of LO catalyst volume. (Note: The engine-out cumulative HC and CO emissions after 100 s are 68.53 and 52.4 mg/ mi, respectively). It is seen that the cumulative HC emissions out of the LO catalyst has a minimum around 1.978 L. (Note: CO cumulative emissions out of the LO catalyst are minimized at around 2.225 L). However, the cumulative emissions out of the main catalyst decrease monotonically (until a critical LO catalyst volume) as we 8461



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Figure 22. Cumulative HC and CO emissions (at 100 s) out of the LO catalyst and main catalyst as a function of the LO catalyst volume for fixed total active metal content. Heating strategy: 150 s of preheating at 0.9 kW followed by 10 s of postheating at 0.9 kW.

decrease the LO catalyst volume. This can be explained as follows. The exhaust gas gets heated in the LO catalyst because of the exotherm during the initial light-off, and this heat is carried downstream by the gas. For the base case (0.8242 L LO catalyst), the main catalyst was heated up during this time and by the time the emission breakthroughs occurred, the main catalyst was already hot enough to effect significant conversions. When the LO catalyst volume increases beyond 2 L, the local active metal concentration in the downstream of the LO catalyst becomes lower and hence the conversion/reaction exotherm is small, leading to higher emissions out of the LO catalyst. Furthermore, the exhaust gas coming out of the LO catalyst is not hot enough to heat up the main catalyst as a large fraction of the heat (sensible heat in the exhaust as well as the reaction exotherm) is consumed in heating up the downstream section of the LO catalyst. Hence the main catalyst is not as active catalytically as in our base case and this leads to higher tailpipe emissions out of the main catalyst. When the LO catalyst volume is decreased below the base case (0.8242 L), the LO catalyst is able to generate high local temperatures (because of the higher local active metal concentration) but the conversion over the LO catalyst is low because of the insufficient contact time. However, this exotherm is able to heat up the main catalyst and the performance of the main catalyst (and the entire system) improves. However, at very low LO catalyst volumes (

Electrically Heated Catalysts for Hybrid Applications: Mathematical

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