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

GTP-06-1037, Shamim, 1

The Effect of Engine Exhaust Temperature Modulations on the

Performance of Automotive Catalytic Converters

Tariq Shamim

Department of Mechanical Engineering

The University of Michigan - Dearborn

Dearborn, MI 48128-1491

Abstract

This paper presents a computational investigation of the effect of exhaust temperature

modulations on an automotive catalytic converter. The objective is to develop a better

fundamental understanding of the converter’s performance under transient driving

conditions. Such an understanding will be beneficial in devising improved emission control

methodologies. The study employs a single-channel based, one-dimensional, non-adiabatic

model. The transient conditions are imposed by varying the exhaust gas temperature

sinusoidally. The results show that temperature modulations cause a significant departure in

the catalyst behavior from its steady behavior, and modulations have both favorable and

harmful effects on pollutant conversion. The operating conditions and the modulating gas

composition and flow rates (space velocity) have substantial influence on catalyst behavior.

Key Words

engine emissions, engine exhaust after-treatment, dynamic behavior, numerical simulations.


Nomenclature

Cgj gas phase concentration of species j, mol/m3

Csj surface concentration of species j, mol/m3

Cpg specific heat of gas, J/(kg·K)

Cps specific heat of substrate, J/(kg·K)

Ga geometric surface area, m2/m3

∆Hk heat of reaction of species k, J/mol

hg heat transfer coefficient between flow and substrate, J/(m2·s·K)

h∞ heat transfer coefficient between substrate and atmosphere, J/(m2·s·K)

kmj mass transfer coefficient for species j, m/s

Rk reaction rate of kth reaction, mol/(m2·s)

Sext external surface to volume area ratio, m2/m3

t time, s

T∞ ambient temperature, K

Tg gas temperature, K

Ts substrate temperature, K

vg gas flow velocity, m/s

z coordinate along catalyst axis, m

Greek symbols

ε void volume fraction

λs thermal conductivity of substrate, J/(m·s·K)

ρg gas density, kg/m3


ρs substrate density, kg/m3

Introduction

Automotive catalytic converters, which are employed to reduce engine exhaust

emissions, are subjected to highly transient conditions during a typical driving cycle. These

transient conditions arise from changes in driving mode (acceleration and deceleration), and

the hysteresis and flow lags of the feedback control system. The transients result in

fluctuations of gas flow rates, compositions and temperatures. These transients make the

catalyst behavior differ significantly from that under steady state conditions [1-5]. Due to its

practical significance, the investigation of the catalyst dynamic behavior has been a subject

of active research for more than two decades. Most prior studies have been focused on the

effects of air-fuel ratio (A/F) and composition modulations. These studies determined that

modulations have both beneficial and harmful effects on pollutant conversion depending

upon the catalyst type, driving mode, exhaust conditions, and pollutants to be removed [4].

In addition to A/F and composition, temperature also has a large influence on the

reaction behavior. Van Neer et al. investigated the effects of a fast periodic change of the

reaction temperature, which may positively affect certain heterogeneously catalyzed

reactions [6]. In this theoretical study, the influence of fast periodic modulations of the

temperature to the preferential oxidation of CO in a hydrogen-rich gas mixture was studied.

The effect postulated to arise by fast periodic changes of the temperature was the generation

of a residual behavior of adsorbed reactant molecules within temperature ranges where they

normally are not adsorbed on the catalyst surface. Therefore, the frequency of the

temperature modulations has to be high enough to prevent desorption of the reactants, but


low enough to allow adsorption and to avoid quasi-steady state by switching the temperature

very rapidly.

The potential benefits of temperature modulations are reported to be higher catalyst

yield owing to the improvement of catalyst coverage and higher selectivity of the catalyst [7].

The imposed modulation frequencies are found to play an important role in determining the

benefits of temperature modulations [7]. However, little attention has been paid to the

investigation of temperature modulation. One potential reason for the limited work in this

area is the difficulty associated with the temperature modulation in conventional reactors.

This is owing to the large thermal masses and the poor surface-to-volume ratio, which restrict

the heat exchange between conventional reactor systems and heating/cooling systems [7,8].

The present study is motivated by realizing the scarcity of extensive investigation of

the effects of temperature modulation on catalysts especially on automotive catalysts. It

investigates the effects of exhaust gas temperature modulations on a three-way automotive

catalyst. The objective is to clearly understand the effect of temperature modulation on the

catalyst performance under engine exhaust conditions.

Mathematical Formulation

The governing equations were developed by considering the conservation of mass,

energy and chemical species. Using the assumptions and notations listed elsewhere [9], the

governing conservation equations for a typical single channel may be written as follows:

The gas phase energy equation:

)()( sgag

g

g

g

Pg TTGhz

Tv

tT

Cg

−−=∂∂

+∂∂

ερ (1)


The gas phase species equations (for 7 species):

)()( js

jga

j

jg

g

jg CCGkm

zC

vt

C−−=

∂∂

+∂

∂ε

(2)

The surface energy equation:

knss

n

ks

ka

sextsgags

ss

Pss

HCCTRG

TTShTTGhzT

tT

C

speciesreaction

∆⋅⋅⋅⋅⋅⋅⋅+

−−−+∂∂

−=∂∂

−

∑=

∞∞

),,,(

)()()1()1(

1

1

2

2

λερε (3)

The surface species equations (for 7 species):

),,,()()1( 1 speciesNsss

ja

js

jga

jj

s CCTRGCCGkmt

C⋅⋅⋅⋅⋅⋅−−=

∂∂

−ε (4)

The superscript j in species equations varies from 1 to 7, representing the following

species: CO, NO, NH3, O2, C3H6, H2 and C3H8. In addition to these species, the kinetic

expressions also include CO2, H2O and N2. The conservation equation for the surface

oxygen storage mechanism is represented by Eq. (4) excluding the convective mass transport

term. The heat and mass transfer coefficients in the above equations are calculated from the

conventional correlations of Nusselt and Sherwood numbers [10]. The chemical reactions

and the corresponding kinetic data used in the present study were similar to those used in

Ref. [9]. The governing equations were discretized by using a non-uniform grid and

employing the control volume approach with the central implicit difference scheme in the

spatial direction. A standard tridiagonal matrix algorithm with an iterative successive line

under relaxation method was used to solve the finite difference equations. The spatial node

size ranging from 0.1693 mm to 19.32 mm and the time step of 0.001 second were employed.

The grid insensitivity of results was ensured by performing a sensitivity study. Details of the

solution procedure are described elsewhere [9].


Results and Discussion

The numerical model’s performance under actual transient driving conditions was

assessed by using experimental measurements across the front catalyst brick from a 4.6L 2V

Lincoln Town car as it was driven during the Federal Test Procedure (FTP-72). The model

results were compared with experimental measurements elsewhere [9]. The model

predictions for the cumulative conversion efficiencies during the FTP cycle were found to be

within 5% of the experimental measurements. The model’s transient performance was

validated by comparing the model’s predictions of instantaneous conversion with the

experimental measurements taken at 1 Hz resolutions. The comparison showed that the

average difference between the model results and measurements was around 30%, which is

within the experimental uncertainties. These validation results showed the suitability of the

model in simulating the transient performance of catalyst.

The catalyst used for the present study was palladium-based and had a length of 3 cm,

cross-sectional area of 86.0254 cm2, cell density of 62 cells/cm2, and wall thickness of

0.1905 mm. The catalyst aging conditions were similar to those of Ref. [10]. The gas mass

flow rate was 1.417x10-2 kg/s with 4.7184x10-5 kg/s CO, 8.0727x10-6 kg/s total HC, and

2.0363x10-6 kg/s NO, and the stoichiometric value of A/F was 14.51. The mean feed gas

temperature was 327oC (600 K). The influence of exhaust gas temperature modulation on

the catalyst dynamic behavior was investigated by considering a steady operating catalyst

suddenly subjected to transient effects. The transient effects were simulated by sinusoidal

modulations of the exhaust gas temperature entering the catalyst. During these oscillations,

other inlet conditions remained unchanged.


Figure 1 shows the results of the imposed modulation near stoichiometric conditions

at 14.7. During the simulations, the exhaust gas temperature, initially set at 600 K, is varied

sinusoidally with a frequency of 0.01 Hz and amplitude of 10%. The low value of

modulation frequency is selected in order to emphasize the effect of imposed modulation

even though it is lower than that of the typical realistic modulations. The effect of other

frequencies, which include more realistic values (such as 0.1 – 5 Hz), is presented in the

following section. With the exhaust gas temperature ranging between 540 K and 660 K, the

selected value of amplitude represents a modest level of modulation. The effect of higher

modulation amplitudes are also considered in the next section. The imposed temperature

modulations cause the catalyst to respond with a time delay.

The results show that CO and HC conversion efficiencies respond sinusoidally to the

imposed sinusoidal temperature modulations. The conversion efficiencies increases

corresponding to an increase in the exhaust gas temperature at the catalyst inlet. After

reaching peak values, the conversion efficiencies decrease corresponding to a decrease in the

exhaust gas temperature. The catalyst periodic response continues as long as the modulating

temperature field is maintained. For the conditions studied, the catalyst NO conversion is

very high (~ 100%) and is not much influenced by the imposed temperature modulations.

The imposed temperature modulation has a negative effect on the CO conversion.

The time-average conversion efficiency, which is obtained by considering the cumulative

pollutant species in and out of the catalyst during the first four hundred seconds, exhibits a

decrease of 3.8% from its steady state value of 82.1% (see Table 1). For these operating

conditions and the CO/HC ratio, HC conversion is low. It is, however, improved by the

imposed modulation. Result listed in Table 1 shows that the HC time-average conversion


efficiency exhibits an increase of 2.5% from its steady state value of 8.71%. The catalyst’s

response is asymmetric with respect to its initial steady state value. With a response

amplitude, which is defined as the difference of peak and trough values of conversion

efficiency, of 44.5%, the modulations have greater influence on CO emission conversion.

The effect on HC emission conversion is relatively low (response amplitude is 19.8%) and

the effect on NO conversion is insignificant (response amplitude is 0.1%).

The effects of temperature modulation in rich and lean regimes are presented next.

The lean regime results, as shown in Figure 2, were obtained by setting the A/F at 17.5. The

catalyst is subjected to sinusoidal modulations in the exhaust temperature with a 0.01 Hz

frequency and 10% amplitude. Under these conditions, CO conversion is high (~100%) and

is not influenced by the imposed modulations. The HC conversion is also high (~90%), but it

is sensitive to the exhaust gas temperature and responds to the imposed modulations with a

large amplitude. The response is periodic, non-sinusoidal, and asymmetric with respect to its

initial steady state value. The modulations result in a decrease in HC conversion

performance as the time-average conversion efficiency is decreased by 6.7% from its steady

state value of 90.55%. The imposed modulations bring about a relatively small response

from NO conversion, which is initially quite low owing to lean conditions. The response is

periodic and non-smooth especially near the peak values. Overall, the effect of modulations

on NO conversion is positive since the time-average conversion efficiency (5.6%) is higher

than the initial steady state value (4.7%).

The rich regime results, as shown in Figure 3, were obtained by setting the A/F at

12.5. The catalyst is subjected to sinusoidal modulations in the exhaust temperature with a

0.01 Hz frequency and 10% amplitude. Under rich conditions, the catalyst is less sensitive to


the imposed temperature modulations. The conversion performance responds to the

modulations with very small amplitudes. Overall, there is no appreciable influence on the

catalyst conversion performance.

Effect of Modulation Amplitude

Figures 4(a) and 4(b) show the effect of imposed modulation amplitude on the

catalyst’s pollutant conversion performance. These figures depict the catalyst conversion

efficiencies as a function of oscillation time period for different amplitudes of exhaust gas

temperature modulations and the response amplitude as a function of the imposed modulation

amplitude. The results are for catalysts, which are operating at A/F of 14.7 and are subjected

to sinusoidal fluctuations in exhaust temperature of 0.01 Hz frequency. The results show that

the increase of oscillation amplitude increases the catalyst’s CO and HC conversion

responses. The NO conversion is not affected by any imposed modulation amplitude, and

remains high at 100%. For all imposed modulation amplitudes, the response amplitude of

CO conversion is the largest. However, the HC conversion is relatively more sensitive to the

changes in the imposed modulation amplitude, as indicated by a steeper slope of the HC

response amplitude curve in Figure 4(b). For all amplitudes, the catalyst response exhibits

steady periodic behavior. For the conditions studied, other than the response amplitude, the

fluctuation amplitude has insignificant influence on any other aspect of the catalyst response.

Effect of Modulation Frequency

Figure 5(a) presents the CO, HC, and NO conversion efficiencies as a function of

imposed modulation time period for different frequencies. All these results are for catalysts,

GTP-06-1037, Shamim, 10

initially operating at exhaust gas temperature of 600 K, and subjected to sinusoidal

modulation in exhaust gas temperature of 10% amplitude. The figure depicts, as expected,

that the catalyst response to imposed oscillation is large at low frequencies and its amplitude

decreases with an increase of the imposed frequency.

The decrease in catalyst response at high frequencies is attributed to progressively

greater neutralization of high frequency fluctuations by diffusion processes over the time

period required to convect them to the reaction sites. After reaching a minimum value, the

influence of frequency on the catalyst response is reversed and the catalyst response to the

imposed modulation starts increasing with an increase of frequency, as shown in Figure 5(b).

The figure shows that the trend of increasing catalyst response with increasing frequency

continues till 20 Hz. Beyond this value, the trend is again reversed and the response starts

decreasing with increasing frequency till 50 Hz, followed by another reversal of trend.

The effect of frequency on the catalyst response can be better understood by plotting

the response amplitude as a function of the imposed frequency, as in Figure 6. The figure

clearly shows that the catalyst response to the imposed temperature modulations is

multimodal. This behavior is different from that caused by the imposed A/F and composition

modulations. In such cases, the response has been reported to be unimodal [11, 12]. This

may be explained by considering that the catalyst’s response to imposed modulations is

mainly determined by diffusion and kinetic processes. Each of these processes has a

different resonant frequency and is dominant in different frequency regimes. The diffusion

processes are generally dominant in the low frequency regime. With the increase of imposed

modulation frequency (the decrease of modulation time scale), the effect of diffusion

processes becomes smaller. At higher frequencies, the response is mainly determined by the

GTP-06-1037, Shamim, 11

kinetic effects. Compared to A/F and composition modulations, the temperature modulations

have stronger influence on kinetic effects and can invoke several response modes at high

frequency regimes resulting in a multimodal response.

Effect of Space Velocity

This section presents the effect of space velocity, which is defined as a ratio of the

gaseous volumetric flow rate at standard temperature and pressure and the volume of the

catalyst, on the catalyst’s response to exhaust gas temperature modulation. In this study, the

space velocity is varied by changing the inlet total gaseous mass flow rate while keeping the

mass flow rates of three major pollutants constant. The study investigates a range of space

velocities from 25,000 hr-1 to 150,000 hr-1, which corresponds to a total gaseous mass flow

rate range of 2.145x10-3 kg/s to 1.289x10-2 kg/s. Since the mass flow rates of major pollutant

species (CO, HC and NO) are fixed, the variations in the space velocity and in the

corresponding total gaseous mass flow rate affect the molar concentrations of pollutant

species, which are different for different space velocity cases. For example, inlet molar

concentrations of pollutant species at 150,000 hr-1 are six times higher than those at 25,000

hr-1. Thus, there are two effects of an increase in space velocity on the pollutant conversion

performance: (i) a reduction in conversion performance owing to reduction is residence time;

and (ii) a variation in conversion performance owing to an increase in pollutant inlet molar

concentrations. The second effect depends on the catalyst operating condition and generally

increases the conversion performance [13]. Hence, the net effect of the variations in space

velocity is the result of these opposing effects.

GTP-06-1037, Shamim, 12

Figure 7 shows the results of the imposed modulation near stoichiometric conditions.

During the simulations, the A/F is set at 14.7 and the exhaust temperature (initially set at 600

K) is varied sinusoidally with a frequency of 0.01 Hz and amplitude of 10%. The

temperature ranges between 540 K and 660 K during each modulation time period. The

catalyst responds to temperature modulation with different amplitudes at different space

velocities.

The results show that the temperature modulation has a strong effect on the catalyst

CO conversion at high space velocities, where the response amplitude is high and the

conversion efficiency fluctuates within a wide range. The response is generally smooth and

periodic. The response amplitude decreases with a decrease in space velocity (see Figure 8).

For 25,000 hr-1–50,000 hr-1, the response is negligible and the catalyst becomes insensitive to

the imposed modulation due to higher residence time.

The results show a significant effect of space velocity on the catalyst HC conversion

response to the imposed temperature modulation. The response amplitude increases with an

increase of space velocity up to 50,000 hr-1(see Figure 8). After reaching a maximum value,

the response amplitude decreases with increasing space velocity. The response is generally

periodic and smooth except at 50,000 hr-1 and 75,000 hr-1, where the response exhibits

somewhat unsmooth behavior, which is owing to numerical convergence problem. NO

conversion remains insensitive to the imposed temperature modulation for all values of space

velocities.

The effects of space velocity on the catalyst’s response to temperature modulation in

lean and rich regimes are presented next. The lean regime results, as shown in Figure 9, were

obtained by setting the A/F at 17.5. The catalyst is subjected to sinusoidal modulations in the

GTP-06-1037, Shamim, 13

temperature with a 0.01 Hz frequency and 10% amplitude. Under these conditions, the CO

conversion is very high (~ 100%) for all space velocities. The conversion is not affected by

temperature modulation at any value in the range of space velocities investigated. The HC

conversion is high at large space velocities. It decreases with a decrease in the space

velocity, which is owing to a corresponding decrease in the inlet molar concentration of HC.

The response amplitude increases with an increase of space velocity from 25,000 hr-1 to

75,000 hr-1 (see Figure 9). Beyond this range, the response amplitude decreases slightly with

an increase of the space velocity. The response is generally smooth and periodic.

Figure 9 shows that the space velocity also affects the NO conversion response to

imposed modulation under lean conditions. As expected, the NO conversion is low at high

space velocities and increases with a decrease of space velocity. The response amplitude is

also affected by the space velocity, but there is no continuous increasing or decreasing trend.

For the space velocity range investigated, the catalyst is most sensitive to the imposed

modulation at 100,000 hr-1. The response is generally unsmooth and non-periodic at several

space velocities.

The rich regime results, as shown in Figure 10, were obtained by setting the A/F at

12.5. The catalyst is subjected to sinusoidal modulations in the temperature with a 0.01 Hz

frequency and 10% amplitude. Under rich conditions, the catalyst conversion performance is

relatively less sensitive to the imposed temperature modulation. Particularly, the CO and HC

conversions are insensitive and remain at their respective steady state values for all space

velocities. Under rich conditions, the NO conversion is high for all space velocities. The

small increase of NO conversion with an increase of space velocity is owing to the

GTP-06-1037, Shamim, 14

corresponding increase of the inlet molar concentration of NO. The NO conversion response

is low and decreases further with an increase of space velocity.

Comparison of Steady State and Transient Results

Figures 11–13 show comparisons of the influence of the space velocity on the catalyst

conversion performance under transient and steady state conditions. For the transient case,

the conversion efficiency shown represents the time-average conversion efficiency, which is

obtained by considering the cumulative pollutant species in and out of the catalyst during the

first 4 cycles.

Figure 11 shows the results near stoichiometric condition. The figure depicts that CO

and HC conversion efficiencies decrease with an increase of space velocity for both steady

state and transient cases. The NO conversion remains mostly unchanged at all space

velocities. The difference in conversion efficiencies of steady state and transient cases is

negligible at most of the space velocities. The modulations have a slightly negative effect on

the CO conversion and a slightly positive effect on the HC conversion.

The results of lean condition are plotted in Figure 12. For these conditions, the space

velocity does not have much influence on CO conversion, which remains high for both

steady state and transient cases. The effect of temperature modulation on the CO conversion

is also negligible for all space velocities. The HC conversion is high at higher space

velocities and decreases with a decrease in the space velocity for both steady state and

transient cases. This decrease is due to the corresponding decrease in the inlet molar

concentration of HC, which has a more dominant negative effect on HC conversion than the

opposing positive effect by an increase in the residence time. The lower HC conversion due

GTP-06-1037, Shamim, 15

to the corresponding increased availability of HC reductants improves the NO conversion at

low space velocities. For 50,000 hr-1 – 75,000 hr-1, the temperature modulation has a strong

positive effect on the NO conversion, which becomes negligibly small at higher space

velocities.

Figure 13 shows the comparison of the catalyst performance under rich conditions.

The results show that the temperature modulation has no appreciable influence on the

catalyst conversion performance for the investigated range of space velocities. At low space

velocities, NO conversion shows a slight increase with an increase of the space velocity,

which is due to corresponding increase of the inlet molar concentration of NO.

Conclusions

In this work, the influence of exhaust gas temperature on the catalyst dynamic

behavior was examined. The transient conditions were simulated by considering the catalyst

subjected to temporal modulation in exhaust gas temperature. The results led to the

following conclusions:

• Under exhaust gas temperature modulations, a three-way catalytic converter exhibits

a significant departure from its steady behavior. The departure is relatively

significant near stoichiometric conditions and for catalyst CO and HC conversion

performance. Near stoichiometric conditions, exhaust gas temperature modulations

result in a decrease of CO conversion and improve HC conversion. The effect on NO

conversion is insignificant.

• Under lean operating conditions, the temperature modulations result in a decrease of

HC conversion and have no significant effect on CO and NO conversions. The

GTP-06-1037, Shamim, 16

modulations also do not have any appreciable effect on conversion performance

under rich operating conditions.

• The effect of imposed modulation frequency on the catalyst’s behavior is multimodal.

At low frequencies, the catalyst’s response is large and its amplitude decreases with

an increase of the imposed frequency. After reaching a minimum value, the response

starts increasing with an increase of frequency. The trend reverses again after

reaching a maximum value. This behavior is different from that caused by the

imposed air-fuel ratio and composition modulations, which depict unimodal response.

• The space velocity significantly influences the catalyst response to the imposed

temperature modulation. The influence is greater at stoichiometric conditions and is

different for different pollutant species.

Acknowledgments

The financial support from the Ford Scientific Research Laboratory, Oak Ridge

National Laboratory and the Center for Engineering Education and Practice (CEEP) of the

University of Michigan-Dearborn is greatly appreciated.

References

[1] Silveston, P. L., 1995, “Automotive exhaust catalysis under periodic operation,”

Catalysis Today, 25, pp.175-195.

[2] Herz, R. K., 1987, “Dynamic behavior of automotive three-way emission control

system,” Catalysis and Automotive Pollution Control, Elsevier, Amsterdam, pp. 427-

444.

GTP-06-1037, Shamim, 17

[3] Herz, R. K., 1981 “Dynamic behavior of automotive catalysts: 1. Catalyst oxidation

and reduction,” Industrial and Engineering Chemistry Product Research and

Development, 20, pp. 451-457.

[4] Silveston, P. L., 1996 “Automotive exhaust catalysis: Is periodic operation

beneficial?,” Chemical Engineering Science, 51, pp. 2419-2426.

[5] Koltsakis, G. C. and Stamatelos, A. M., 1999, “Dynamic behavior issues in three-way

catalyst modeling,” AIChE Journal, 45, pp. 603-614.

[6] Van Neer, F. J. R., Kodde, A. J., Den Uil, H., and Bliek, A., 1996, “Understanding of

resonance phenomena on a catalyst under forced concentration and temperature

oscillations,” Canadian Journal of Chemical Engineering, 74, pp. 664–673.

[7] Brandner, J. J., Emig, G., Liauw, M. A., and Schubert, K., 2004, “Fast temperature

cycling in microstructure devices,” Chemical Engineering Journal, 101, pp. 217–224.

[8] Silveston, P. L., and Hudgins, R. R., 2004, “Periodic temperature forcing of catalytic

reactions,” Chemical Engineering Science, 59, pp. 4043–4053.

[9] Shamim, T., Shen, H., Sengupta, S., Son, S. and Adamczyk, A. A., 2002, “A

comprehensive model to predict three-way catalytic converter performance,” ASME

Journal of Engineering for Gas Turbines and Power, 124, pp. 421-428.

[10] Montreuil, C. N., Williams, S. C. and Adamczyk, A. A., 1992, “Modeling current

generation catalytic converters: Laboratory experiments and kinetic parameter

optimization – steady state kinetics”, SAE Paper No. 920096.

[11] Shamim, T. and Medisetty, V. C., 2003 “Dynamic response of automotive catalytic

converters to variations in air-fuel ratio,” ASME Journal of Engineering for Gas

Turbines and Power, 125, pp. 547-554.

GTP-06-1037, Shamim, 18

[12] Shamim, T., 2005, “Dynamic behavior of automotive catalytic converters subjected to

variations in engine exhaust compositions,” International Journal of Engine Research,

6, pp. 557-568.

[13] Shamim, T., 2005, “The effect of space velocity on the dynamic characteristics of an

automotive catalytic converter,” SAE Transactions – Journal of Fuels and Lubricants,

114(4), pp. 974-982.

GTP-06-1037, Shamim, 19

Figure Captions

Figure 1. Catalyst response to sinusoidal modulations in exhaust gas temperature near

stoichiometric operating conditions (A/F = 14.7, Mean exhaust temperature = 600

K, Frequency = 0.01 Hz, Amplitude = 10%)

Figure 2. Catalyst response to sinusoidal modulations in exhaust gas temperature under lean

operating conditions (A/F = 17.5, Mean exhaust temperature = 600 K, Frequency

= 0.01 Hz, Amplitude = 10%)

Figure 3. Catalyst response to sinusoidal modulations in exhaust gas temperature under rich



Figure 4. Catalyst response to sinusoidal modulations in exhaust gas temperature: Effect of

modulation amplitudes on conversion efficiencies. (a) Conversion efficiencies as

a function of oscillation time period; (b) Catalyst response amplitude as a function

of the imposed modulation amplitude (A/F = 14.7, Mean exhaust temperature =

600 K, Frequency = 0.01 Hz)


modulation frequencies on conversion efficiencies. (a) Frequency range = 0.01 Hz

– 1 Hz; (b) Frequency range = 1 Hz – 100 Hz (A/F = 14.7, Mean exhaust

temperature = 600 K, Amplitude = 10%)


modulation frequencies on conversion response amplitudes (A/F = 14.7, Mean

exhaust temperature = 600 K, Amplitude = 10%)

GTP-06-1037, Shamim, 20

Figure 7. Catalyst response to sinusoidal modulations in exhaust gas temperature near

stoichiometric operating conditions for different space velocities (A/F = 14.7,

Mean exhaust temperature = 600 K, Frequency = 0.01Hz, Amplitude = 10%)

Figure 8. Catalyst response to sinusoidal modulations in exhaust gas temperature for

different space velocities (Mean exhaust temperature = 600 K, Frequency =

0.01Hz, Amplitude = 10%)

Figure 9. Catalyst response to sinusoidal modulations in exhaust gas temperature under lean

operating conditions for different space velocities (A/F = 17.5, Mean exhaust

temperature = 600 K, Frequency = 0.01Hz, Amplitude = 10%)

Figure 10. Catalyst response to sinusoidal modulations in exhaust gas temperature under rich

operating conditions for different space velocities (A/F = 12.5, Mean exhaust

temperature = 600 K, Frequency = 0.01Hz, Amplitude = 10%)

Figure 11. Catalyst conversion efficiency for different space velocities in steady state and

transient (exhaust gas temperature modulation) conditions: near stoichiometric




transient (exhaust gas temperature modulation) conditions: under lean operating

conditions (A/F = 17.5, Mean exhaust temperature = 600 K, Frequency = 0.01 Hz,

Amplitude = 10%)


transient (exhaust gas temperature modulation) conditions: under rich operating

GTP-06-1037, Shamim, 21

conditions (A/F = 12.5, Mean exhaust temperature = 600 K, Frequency = 0.01 Hz,

Amplitude = 10%)

GTP-06-1037, Shamim, 22

Table 1 Comparison of time-average conversion efficiencies for

exhaust gas temperature modulations (Modulation amplitude = 10%,

Mean exhaust gas temperature = 600 K)

Frequency (Hz)

CO (%)

NO (%)

HC (%)

0.01 78.34 99.94 11.22

0.05 81.38 99.94 9.23

0.1 82.59 99.94 9.10

0.5 82.55 99.95 8.91

1 82.43 99.95 8.86

Stoichiometric (A/F = 14.7)

Steady state 82.1 99.90 8.71

0.01 99.86 5.64 83.82

0.05 99.90 5.83 89.95

0.1 99.92 5.22 91.55

0.5 99.91 4.59 92.04

1 99.91 4.56 92.03

Lean (A/F = 17.5)

Steady state 99.9 4.70 90.55

0.01 0.02 92.40 5.05

0.05 0.01 92.41 5.03

0.1 0.01 92.38 5.03

0.5 0.01 92.38 5.04

1 0.01 92.38 5.04

Rich (A/F = 12.5)

Steady state 0 92.40 5.04

GTP-06-1037, Shamim, 23

0

20

40

60

80

100

0

20

40

60

80

100

40

60

80

100

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0520

560

600

640

680

720

CO

HC

Con

vers

ion

Eff

icie

ncy

(%)

NO

Gas temperature at the catalyst inlet

Catalyst's average bed temperature

Tem

pera

ture

(K

)

Time Period

Figure 1

GTP-06-1037, Shamim, 24

40

60

80

100

0

20

40

60

80

100

0

20

40

60

80

100

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0520

560

600

640

680

720

0 1 2 3 40

5

10

15

CO

HC

Con

vers

ion

Eff

icie

ncy

(%)

NO



Tem

pera

ture

(K

)

Time Period

Figure 2

GTP-06-1037, Shamim, 25

0

20

40

60

80

100

0

20

40

60

80

100

0

20

40

60

80

100

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0520

560

600

640

680

720

0 1 2 3 490

92

94

CO

HC

Con

vers

ion

Eff

icie

ncy

(%)

NO



Tem

pera

ture

(K

)

Time Period

Figure 3

GTP-06-1037, Shamim, 26

0

20

40

60

80

100

0

20

40

60

80

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.090

95

100

105

CO

10% 15% 20% 25%

HC

Con

vers

ion

Eff

icie

ncy

(%)

NO

Time Period

Figure 4 (a)

GTP-06-1037, Shamim, 27

10 15 20 25

0

20

40

60

80

100

Res

pons

e A

mpl

itud

e (%

)

Modulation Amplitude (%)

CO HC NO

Figure 4 (b)

GTP-06-1037, Shamim, 28

50

60

70

80

90

100

0

10

20

30

40

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

90

95

100

105

110

CO

1 Hz 0.5 Hz 0.1 Hz 0.05 Hz 0.01 Hz

HC

Con

vers

ion

Eff

icie

ncy

(%)

NO

Time Period

Figure 5 (a)

GTP-06-1037, Shamim, 29

75

80

85

90

0

5

10

15

20

25

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

95

100

105

CO

1 Hz 5 Hz 10 Hz 20 Hz 50 Hz 100 Hz

HC

Con

vers

ion

Eff

icie

ncy

(%)

NO

Time Period

Figure 5 (b)

GTP-06-1037, Shamim, 30

0.01 0.1 1 10 100

0

10

20

30

40

50

Res

pons

e A

mpl

itud

e (%

)

Frequency (Hz)

CO HC NO

Figure 6

GTP-06-1037, Shamim, 31

0

20

40

60

80

100

0

20

40

60

80

100

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

60

80

100

CO

space velocity = 25000 hr-1





HC

Con

vers

ion

Eff

icie

ncy

(%)

NO

Time Period

Figure 7

GTP-06-1037, Shamim, 32

0

20

40

60

80

0

20

40

60

80

25,000 50,000 75,000 100,000 125,000 150,000

0

20

40

60

80

CO

stoichiometric case lean case rich case

HC

Res

pons

e A

mpl

itud

e (%

)

NO

Space Velocity

Figure 8

GTP-06-1037, Shamim, 33

60

80

100

0

20

40

60

80

100

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

0

20

40

60

80

100

CO






HC

Con

vers

ion

Eff

icie

ncy

(%)

NO

Time Period

Figure 9

GTP-06-1037, Shamim, 34

0

10

20

30

40

50

0

10

20

30

40

50

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0

50

60

70

80

90

100

CO






HC

Con

vers

ion

Eff

icie

ncy

(%)

NO

Time Period

Figure 10

GTP-06-1037, Shamim, 35

50

60

70

80

90

100

0

10

20

30

40

50

25,000 50,000 75,000 100,000 125,000 150,000

50

60

70

80

90

100

CO

transient case steady state case

HC

Con

vers

ion

Eff

icie

ncy

(%)

NO

Space Velocity

Figure 11

GTP-06-1037, Shamim, 36

50

60

70

80

90

100

0

20

40

60

80

100

25,000 50,000 75,000 100,000 125,000 150,0000

20

40

60

80

100

CO


HC

Con

vers

ion

Eff

icie

ncy

(%)

NO

Space Velocity

Figure 12

GTP-06-1037, Shamim, 37

0

10

20

30

40

50

0

10

20

30

40

50

25,000 50,000 75,000 100,000 125,000 150,00050

60

70

80

90

100

CO


HC

Con

vers

ion

Eff

icie

ncy

(%)

NO

Space Velocity

Figure 13

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

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