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2009-01-1285

Automated model fit tool for SCR control and OBD development

Edwin v.d. Eijnden, Robert Cloudt, Frank Willems, Peter v.d. Heijden TNO Automotive

Copyright © 2009 SAE International

ABSTRACT

Reaching EUROVI Heavy Duty emission limits will result in more testing time for developing control and OBD algorithms than to reach EUROV emissions. It is likely that these algorithms have to be adapted for a WHTC (World Heavy Duty Transient Cycle) for EUROVI. This cycle when started cold can only be performed a limited times a day on the engine testbench, because of the cooling down time. The development time and cost increases to reach EUROVI emission levels. Accurate simulation tools can reduce the time and costs by reducing the amount of tests required on the testbench. In order to use simulation tools to develop pre calibrations, the models must be fitted and validated. This paper will focus on the fit process of an SCR (Selective Catalytic Reduction) model. A unique test procedure has been developed to characterize an SCR catalyst using an engine testbench in ±2 days. This data is used in an automatic SCR fit tool to obtain the model parameters in a few days. The result is a model that predicts the NO, NO2 and NH3 SCR out concentration accurately. The fitted SCR model can predict tailpipe NOx emissions for a wide range of test cycles within 10 % (see Table 5). The validated model is used to develop and calibrate SCR control and OBD algorithms.

INTRODUCTION

Obtaining the EUROVI emission limit will be the new challenge after the EUROV emission limits for the European market. Cold start, OBD legislations [1] and In Use Compliance demands will ask for robust control algorithms and intensive testing.

The world-harmonized test cycles WHTC (World Harmonized Transient Cycle) will most likely be used for EUROVI. This cycle can start with a hot and a cold start. A limit on the NO2 in the NOx emissions can be introduced later. OBD and In Use Compliance limits will become more stringent. It can be seen that the NH3 stored in the SCR catalyst is an important factor that becomes more important for Zeolite catalysts and cold start cycles. The storage capacity is typically much higher for a Zeolite than for a Vanadium catalyst, especially at low temperatures. In Figure 1 [7] the NH3 storage capacity of a SCR catalyst is plotted as a function of the catalyst temperature at NSR (Normalized Stoichiometric Ratio) 1.2. NSR is the ratio between NH3 and NOx molecules pre SCR.

Figure 1: NH3 storage capacity SCR catalyst as a function

of temperature

0

0.2

0.4

0.6

0,8

1

1.2

1.4

1.6

200 300 400 500

Tcat (°C)

NH3

(g/l) Vanadium steady-state storage Zeolite steady- state storage

The NOx conversion capability of the SCR catalyst depends on the NH3 that is stored on the catalyst surface and depends on the catalyst temperature (see Figure 2). Figure 2 was generated by performing simulations with the in this paper described SCR model. In the figure the NOx conversion is plotted as a function of the NH3 storage at a constant catalyst temperature. The simulation is realized with the conditions pre SCR (see Table 1) and a SCR catalyst volume of 34.1 l.

Table 1: Conditions NH3 storage simulations

Tcat (°C) 200 250 300

Exhaust flow (kg/s) 0.4 0.4 0.4

NO (ppm) 200 200 200

NO2 (ppm) 200 200 200

For each temperature the urea injection pre SCR is increased in small steps. At each urea injection quantity steady state conditions are obtained before determining the NH3 storage quantity and the NOx conversion. These values are plotted in Figure 2. At a catalyst temperature of 200°C a significant amount of NH3 is stored on the catalyst surface to obtain a NOx conversion higher than 80 %. At a catalyst temperature of 250°C and 300°C a high NOx conversion is realized with a much lower NH3 storage quantity.

0

10

20

30

40

50

60

70

80

90

100

0 0.02 0.04 0.06 0.08 0.1

Tcat=200 °CTcat=250 °CTcat=300 °C

Figure 2: NOx conversion as a function of NH3 storage

The NH3 storage capacity reduces at higher temperatures. A high NH3 storage at low temperatures will be released by the catalyst at higher temperatures. The result can be a NH3 slip post SCR that is higher than 10 ppm. To obtain the maximum NOx conversion potential over a cold start WHTC, the NH3 storage during the cycle should be optimized while maintaining within the NH3 slip limits. Controlling the NH3 storage in the SCR catalyst during cycles to obtain an optimum NOx conversion while keeping the NH3 slip post SCR below 10 ppm, takes a lot of expensive testing time. In the control development process, models and simulation tools become more important [2],[3]. The

model needs to describe the important reactions. In case of the SCR model the reactions are the reaction of NO and NO2 with NH3. The NH3 storage at the catalyst surface is an important state of the model as well. An SCR model can be used to develop and calibrate control strategies optimally for test cycles as a WHTC. The calibration can than be realized in days instead of more than a week. The SCR model must predict the NOx SCR out emissions for this purpose within 10 %. The NH3 slip in steady state points must be predicted in the same order. The NH3 slip peaks during temperature transient should be predicted qualitatively. A fit and validation of the SCR model on test data is required to obtain this accuracy. This paper will mainly focus on a model based algorithm development process for SCR. An automated fit tool for the SCR model is an important part of the process.

NH3 storage (g/l)

NOx conv. (%)

OUTLINE MODEL BASED DEVELOPMENT

The model based development process for diesel aftertreatment control development used by TNO is shown schematically in Figure 3. The process starts with available hardware to test the SCR performance on NOx reduction. The process ends with a statement about the feasibility and/or a demonstrator that meets a defined emission legislation. In order to implement the control strategy in a production vehicle the strategy must be translated to production software. Furthermore the strategy has to be tested in all circumstances that can occur in the vehicle.

Figure 3: SCR control development process using

validated models

The process consists of the following steps: 1. Catalyst

Develop test set-up with SCR catalyst 2. Engine dyno tests

Characterize SCR catalyst on engine testbench. The

tests can be realized in ±2 days when the test set-up is working properly.

3. Fit tool Fit SCR model parameters with dedicated tool

4. Simulation model Perform simulations with SCR model and simple urea dosing strategy to predict potential emission reduction

5. Control algorithm development Develop control strategy for urea dosing quantity in a simulation environment. The pre-calibration of the strategy can be realized in the simulation environment.

6. Control prototyping Prepare control strategy for implementation on rapid control prototyping device

7. Calibration Test and calibrate urea dosing control strategy on engine testbench or chassis dynamometer

8. Testing Measure tailpipe NOx emissions over a test cycle using an engine testbench or chassis dynamometer.

The order of the steps is executed as described above. Some of the steps must be iterated, for example when a new SCR catalyst is selected step 2 must be repeated. Also the steps after 2 must be repeated. If phenomena are measured during validation that are not incorporated in the model, but are important for the control strategy, the model model must be expanded.

The gain of using the fitted models in the process is that the time required for steps 7 and 8 can be reduced significantly. By using the fitted simulation model, the control strategy can be tested to a large extend using simulation. Pre-calibration can reduce the calibration time on the engine test bench or chassis dynamometer. Especially when cold start cycles are required for the calibration, the calibration time can be realized in a couple of days instead of more than a week.

MODEL

The SCR model is described in earlier papers [4],[5]. It describes urea decomposition (1), hydrolysis (2) standard SCR reaction (3), fast SCR reaction (4) slow SCR reaction (5), NH3 oxidation to NO (6) and NH3 oxidation to N2 (7): H4N2CO → NH3 + HNCO (1) HNCO + H2O → NH3 + CO2 (2) 4NH3 + 4NO + O2 → 4N2 + 6H2O (3) 4NH3 + 2NO + 2NO2 → 4N2 + 6H2O (4)

4NH3 + 2NO2 + O2 → 3N2 + 6H2O (5) 4NH3 + 5O2 → 4NO + 6H2O (6) 4NH3 + 3O2 → 2N2 + 6H2O (7) The model also describes the storage of NH3 on the catalyst surface. The model is capable of predicting the steady state (temperature, exhaust flow, urea, emissions etc.) as transient performance of the catalyst. Table 2 shows information about the model and Table 3 the in- and outputs. The model is a 1D model (see Figure 4). The catalyst is divided in slices in longitudinal direction. For each slice all the above reaction equations are solved.

Figure 4: 1D SCR catalyst model

By solving algebraic loops the model is highly optimized for speed. Solving the algebraic loop improves the

Slice

SCR in SCR out

Catalyst Engine dynotest Fit tool

Simulation model

Testing Calibration Control prototyping

Control development

numeric stability and allows a bigger step size in the calculation. A fixed step solver is used with a step size of ≤0.1 s. The fixed step solver was required for using the model for real-time applications as Hardware In the Loop and in control strategies.

Table 2: SCR model implementation

SCR model

Type of model 1D

Step size ≤0.1 s

Solver Fixed step solver

Simulation environment Matlab/Simulink

Table 3: SCR model in- and outputs

Inputs States Outputs

H4N2CO (urea) NH3 storage H4N2CO (urea)

HNCO HNCO

NO NO

NO2 NO2

NH3 NH3

Exhaust flow

Exhaust flow

Temperature

Temperature

Pressure

Pressure

The SCR model consists of 3 main parts: thermal model, pressure drop model and model with reaction kinetics (see Figure 5).

Figure 5: Schematic overview SCR model

The SCR model is a phenomenological model. The model contains reaction rates for the NH3 reaction with NO, NO+NO2, NO2, NH3 adsorption and desorption, NH3 oxidation and urea decomposition. Each reaction has a reaction rate. The reaction rates are described in more detail in [4] and in the appendix. The reaction rate of the NO reaction (see Equation 1) is for example described as follows: Equation 1: NO reaction rate (rno)

−⋅⋅⋅⋅=

−−*

3

3

31*0 NH

NH

s

NO

eCekr NHNO

RT

E

NONO

θ

θ

θ

Parameters 0

NOk Pre-exponential coefficient standard reaction

[m3/mol.s]

NOE Activation energy standard reaction [J/mol]

R Universal gas constant [J/mol.K] *

3NHθ Critical NH3 surface coverage [-]

Variables

NOC Concentration NO in exhaustgas [mol/m3]

3NHθ NH3 surface coverage [-]

sT Temperature solid phase catalyst [K]

In this equation the gas constant is fixed, but kNO, ENO

and θ*NH3 must be fitted based on experiments. Pre-defined experiments can be executed on a flow bench or an engine testbench [6]. Based on the data the model parameters are fitted.

MODEL FIT PROCESS

FLOW BENCH MEASUREMENTS A flow bench is often used to obtain the required measurement data to fit the reaction kinetic parameters (see Figure 6). With the flow bench the catalyst efficiency at every gas composition, temperature and flow can be tested. This makes it possible to isolate specific reactions. In case of the NO standard reaction (1), a gas can be fed through the brick of NO without NO2. The fitting process of the standard reaction rate parameters is an optimization with only 2 parameters to optimize

(0

NOk and NOE ).

Figure 6: Flow bench set-up

MFC

MFC

N2

NH3

O2

CO2

NO2

NO

HNCOSampling point

Gas Analysis

NH3

H2O

MFC

MFC

MFC

NONO2

+Urea2

H O

Preheater

Entrained

Flow Reactor

Kiln-2

MFC

O2

Kiln-1

cat

solids feed

possibility

Thermal model

Pressure model

Reaction kinetics

Inputs Outputs

The model fitted based on this data is validated with ETC (European Transient Cycle) testdata from an SCR set-up on an engine testbench. Although the same catalyst material was used, the error in the prediction of the SCR catalyst NOx conversion over the cycle was >10 %. One of the reasons for lower accuracy is that other gases in the exhaustgas have an effect on the NOx conversion potential (positive as well as negative). These effects were not all studied in the flow bench. The accuracy of the model can possibly be improved by expanding the model with more reaction equations. Adding more reaction equations will make the model more complex, use more simulation time and will create more parameters to fit. This kind of detailed model is required to fully understand all the reaction principals on the catalyst surface. For control purposes it is important to get a good steady state and transient relation between the urea dosing quantity and the NOx/NH3 SCR out. This relation can possibly be realized with a less detailed model and fitting the model with a gas composition that is very similar to the final application. The result of this approach is shown in this paper. If the model accuracy is still not sufficient for it’s purpose the model must be expanded with more reaction kinetics. A reason to use one test set-up for characterization and validation is as follows: During the characterization, validation and final implementation the gas composition are very similar when using for both phases an engine testbench. The difference in gas composition is much bigger when using a flowbench for characterization and a testbench for validation. In case the model neglects the influence of certain emissions on the NOx conversion potential of the SCR catalyst, this can give an error during the validation. If the NOx reaction parameters are fitted with a gas composition that is very similar between characterization and implementation, the error in the NOx conversion can possibly be decreased. Therefore a test sequence and parameter optimization methodology was developed to fit the model parameters using engine testbench measurements. The test sequence and the parameter optimization methodology are described in the next two paragraphs. ENGINE TESTBENCH MEASUREMENTS The following engine test bench set-up was used for the SCR characterization measurements (see Figure 7). If a DOC (Diesel Oxidation Catalyst) and DPF (Diesel Particulate Filter) is used in the final exhaustline configuration, it is also used during the tests. This has the following advantages:

• Not a special test set-up has to be developed for the SCR characterization.

• Sufficient NO2 is required pre-SCR to properly fit the fast (4) and slow (5) NOx reaction rates.

• The emission composition is closer to the final use of the system

Urea is injected before the SCR catalyst. The AMOX if used behind the SCR catalyst should be removed for the measurements or the emissions should be measured behind the SCR catalyst. The NO, NO2 and NH3 emissions SCR out are required to fit the SCR model.

Figure 7: Engine testbench set-up for characterizing SCR

catalyst

The emissions shown in Table 4 are measured pre and post the SCR catalyst. Table 4: Measured emissions

Measured pre SCR post SCR ambient

NO X X

NO2

X X

NH3 X

Exhaust flow X

Temperature X X X

Pressure X X X

Humidity X

FIT PROCESS Fitting the parameters implies optimizing the parameters based on the error between the measured and simulated SCR out emissions. The experiments for characterizing the SCR catalyst should isolate a subset of model parameters as much as possible. Examples of subsets are shown later in this paragraph. Furthermore the data should be rich enough to uniquely find a fit for a specific parameter. When an emission cycle (for example European Stationary Cycle) is used to optimize all model parameters at the same time, it will possibly not find a unique solution. With the engine testbench set-up described in the previous paragraph the characterization of the SCR catalyst is realized. For the characterization a series of predefined measurements have to be performed. The measurements can be realized in less than 3 days if the test set-up is working properly. The measurements are the input of a special developed dedicated fit tool for the SCR model. The tool uses the data measured pre and post SCR catalyst as an input for the fit process. The tool was developed to reduce the time needed for the fit process. The first time the model was fitted based on the data the process took more than a month. The fit

oxidation catalyst (if used)

soot filter (if used)

SCR catalyst

Measurement point pre-SCR

Measurement point post-SCR

urea injection Ambient measurement of temperature, absolute pressure and humidity

engine engine brake

throttle valve

tool was developed to automate the fit procedure and reduce the fit process to a few days. The SCR model parameters are fitted with the following steps: 1. Fit thermal model 2. Fit pressure drop model 3. Fit NH3 adsorption and desorption model 4. Fit NO and NO2 conversion model 5. Fit NH3 oxidation The parameters that can be measured directly for example the dimensions of the catalyst and the density of the brick, are filled in the tool as constants. ANALYSES FIT PARAMETERS The following parts of the SCR model are described in more detail in [4]. The different steps in the fit process are briefly described below: 1 Thermal model (2 parameters) Fitting the thermal model (see Appendix) is the first step in the process. This fit can be realized using cycle data (for example ESC). During the test the temperature pre and post the SCR catalyst is measured. The heat transfer gas->solid and solid ambient are fitted based on the error between the simulated and the measured catalyst out temperature. The total quadratic error is minimized. 2 Pressure drop model (1 parameter) The pressure drop model can be fitted based on the same cycle. Figure 8: Example NH3 adsorption and desorption

experiment

The pressure is measured pre and post SCR catalyst. The post SCR pressure is used as an input for the simulation.The total quadratic error between pre SCR pressure simulated and measured is minimized with

fitting hF (see Appendix).

3 NH3 adsorption and desorption model (5 parameters)

The following parameters: 0

ak ,0

dk ,0

dE ,Ω andα have

influence on the NH3 storage (see Appendix). The fit procedure for the NH3 adsorption and desorption parameters require an estimate of the NH3 that is stored in the catalyst. The amount of NH3 in the catalyst can be estimated by integrating the following balance. The NH3 that enters the catalyst minus the amount of NH3 that exits the catalyst and minus the NH3 that is used for the NOx conversion reaction. The NOx converted is NOx SCR in minus NOx SCR out. This assumes that limited NH3 is oxidized. This statement is only valid for low temperatures (below 350 °C). The NH3 SCR in is calculated by assuming that all urea injected is converted to NH3. The bases of fit methodology is performing step tests in the urea dosing at different dosing quantities, temperatures and Space Velocities (residence time of the gas in the catalyst). The steps in the urea are performed while maintaining a constant engine work point (steady state temperature and flow conditions). Figure 8 shows the results of the urea step measurements performed at a 200 °C operating temperature of the SCR catalyst.

Catalyst temperature of 200 °C

The lowest part of the graph shows the NSR steps performed. These NSR steps are performed at various operating temperatures of the SCR catalyst. In order to get an accurate estimate of the NH3 storage, the NOx and NH3 SCR in and out have to be measured very accurately. A small error in the signals can be integrated to give drift in the NH3 storage estimate. The following parameters are optimized based on the

data 0

ak ,0

dk ,0

dE ,Ω andα .

After the fit process the same experiment is simulated using the fitted parameters. The result is shown in Figure 8. The figure shows the NO, NO2 and NH3 SCR out simulated and measured. The Space Velocity is defined as the exhaust mass flow divided by the catalyst volume and a standardized density of the exhaust flow. The formula is shown in (Equation 2): Equation 2: Definition Space Velocity

)/( tan, dardsexhaustcatwexhaustflo VmSV ρ⋅=

SV Space Velocity 1/h mexhaustflow exhaust massflow kg/h Vcat catalyst volume m

ρexhaust,standard normalised rho (1.28) for SV computation (0

oC, 1013 mbar) kg/m

3

Figure 9: Fit results NO and NO2 reactions using 13 steady

state operating points

4 NO and NO2 model (6 parameters) The NO and NO2 reaction equations are fitted using steady state engine work point data with urea dosing. In this optimization 13 operating points were selected with different temperatures and Space Velocities (SV). With the SCR fit tool the following fit was found for the 13 operating points (see Figure 9). 5 NH3 oxidation model (4 parameters) At high SCR catalyst temperatures (above 350 °C) a part of the NH3 is oxidized. A part is oxidized to NO and a part to N2. The NOx conversion reactions should be fitted first before the oxidation reaction to NO and N2 can be fitted. The following parameters (see Appendix):

0

oxnok , 0

oxnoE ,

0

2oxnk and 0

2oxnE

are optimized to fit the oxidation to NO and N2.

VALIDATION

Figure 10: Validation of the fitted SCR model using

measured ESC data

HEAVY DUTY CYCLES Finally the model fit is validated using an European Stationary Cycle (ESC). The emissions pre and post SCR catalyst are measured when injecting an urea amount resulting in a NH3 slip varying between 0 and 100 ppm. The pre SCR emissions are used as an input for the simulation, the simulation results post SCR are compared with the measurements. The results of the validation are shown in Figure 10. The result shows the measured pre NO and NO2 emissions in green. The simulated (red line) and measured (blew line) post SCR NO, NO2 and NH3 emissions are plotted in the same figure. The NO and NO2 are predicted well steady state as well as transient. The NH3 slip is predicted well qualitatively and even quantitatively the prediction of the NH3 slip peaks are quite well. LIGHT DUTY CYCLES The SCR model fit process is also applied for light duty purposes. For the model validation MVEG, ARTEMIS and US06 driving cycles were performed on a chassis dynamometer.

The measurements were realized with a vehicle with the same engine and exhaustline that was used on the engine testbench for the characterization. The emissions were measured during these tests pre- and post SCR. The pre-SCR emissions were used for simulation with the fitted SCR model. The simulated post SCR emissions were compared with the measured emissions for validation. The simulations were realized with one parameterset for the SCR catalyst. The accuracy of the NOx conversion prediction of the model for MVEG, ARTEMIS and US06 cycles are shown in Table 5. The error of the simulated NOx conversion compared to the measured NOx conversion is defined as follows: 100x(NOx measured

* - NOx simulated

*) / NOx measured

*

*NOx measured and simulated are the NOx conversion over the entire cycle The 0 % error on the MVEG cycle was realized by slightly fine tuning the parameters for this cycle. The results for the other cycles were realized with the same parameters as used for the simulation of the MVEG cycle. Table 5: Relative error NOx conversion prediction

Cycle relative error NOx conversion

MVEG 0 %

Artemis 6 %

US06 3 %

CASE STUDIES

SIMULATION SET-UP The SCR model is used in the following simulation set-up (see Figure 11). The simulation set-up requires pre-SCR emission traces measured on an engine test bench or chassis dynamometer. The emission traces are used as an input for the SCR model. The amount of urea that is dosed during the cycle can be a measured trace or a control strategy that is simulated as well. An OBD strategy can be simulated in the same environment.

Figure 11: Simulation set-up for testing urea control or

OBD algorithm

OBD SOFTWARE DEVELOPMENT The SCR fit procedure and model is used to create an environment to test OBD algorithms for a customer. A fresh SCR catalyst was characterized on the engine testbench and the data was used to fit the SCR model. The catalyst was aged to different ageing stages and at each stage the catalyst was characterized and fitted. This resulted in a parameterset for each ageing stage. The NOx conversion for each catalyst ageing stage could be predicted within 10 % for MVEG, ARTEMIS and US06 (see Table 5). With the SCR model and parametersets simulation are being performed for OBD algorithm development and testing. UREA DOSING STRATEGY Another important use for the model and the fit routine is control development. The model is being used for developing closedloop control strategies based on NOx and NH3 SCR out. For low temperature cycles and Zeolite catalysts information about the NH3 that is stored in the SCR catalyst proved important [7],[8] and [9]. At low temperature (below 300 °C), the NH3 storage capacity of a Zeolite is significant. Figure 8 shows the NOx and NH3 SCR out when switching the urea injection on and off. These step tests are performed for NSR values from 0.2

to 4. With this test the NH3 storage in the SCR catalyst is determined at different temperatures. It takes at least minutes at 200 °C before steady state is reached. The NH3 stored at low temperature can be released quickly when the temperature of the SCR catalyst increases rapidly and can result in NH3 slip. In order to prevent NH3 slip the NH3 storage is controlled to a reference value at different temperatures [7]. For this purpose the SCR model is implemented in a real-time control strategy to predict the NH3 storage. The dosing strategy can increase the NH3 storage by injecting more urea than used for the NOx reduction or reduce the NH3 storage by injecting less than the quantity that is used for the NOx reduction. This way NH3 slip can be prevented [7]. The model is fitted with the test procedure and fit tools described in this paper. The real-time model can also be used to predict the NOx conversion more accurately under varying NO/NO2 ratios than using a NSR table. The NSR table contains the NSR as a function of Space Velocity (SV) and catalyst temperature (see Figure 12). This table is determined using steady state measurements at different temperatures and flows. The NSR is determined at each steady state point to result in for example 10 ppm NH3 slip post SCR.

Figure 12: NSR table resulting in 10 ppm NH3 slip

The NSR table values depend on the NO/NO2 ratio [10]. The table can for example be determined using a flow bench with NO/NO2 ratio 1. When a Diesel Oxidation Catalyst is used before the SCR catalyst, the NO/NO2 ratio is however not constant. It will depend on the Space Velocity and the catalyst temperature. The NOx conversion of the catalyst can be predicted more accurately by calibrating the NSR table on the engine testbench with the final exhaustline configuration. This way the NSR value is calibrated for the actual NO/NO2 ratio at each catalyst temperature and space velocity. The table resulting from these measurements represents the NSR for steady state conditions. When the temperature in the exhaustline is not steady state the DOC and SCR temperature can have different values than when measured during steady state.

Control/ OBD

Validated

SCR model

Urea dosing

10 ppm NH3 slip

Test vectors

(engine

measurements

ESC, ETC, WHTC,

MVEG etc.)

NOx [g/kWh] or [g/km] PM, Temperatures, Pressures, NH3 slip [ppm]

The NOx conversion dependency on the NO/NO2 ratio can be predicted with the real-time SCR model. This approach requires also a real-time model of the DOC. The DOC model predicts the NO/NO2 ratio pre SCR. The real-time SCR model predicts the NOx conversion based on this ratio and the NH3 oxidation. This approach can be used instead of a NSR table. The SCR model reduces the required amount of measurements compared to the NSR table calibration because no full factorial measurements have to be performed as a function of temperature and Space Velocity. For example an NSR table of 10x10 requires 100 steady state measurements. The NOx reaction rates in the SCR model can be calibrated with 13 steady state engine working points. The model environment is currently being expanded with models and fit tools for DOC, DPF (Diesel Particulate Filter) and AMOX (Ammonia Oxidation Catalyst). The simulation environment will also be used in the future for In-Use Compliance testing in combination with an altitude and climate chamber.

CONCLUSION

An accurate SCR model is a valuable tool for developing and calibrating EUROVI control and OBD strategies. In order to predict the SCR out emissions accurately, the model must be fitted based on measurement data. This can take more than a month without using the appropriate tools. This paper describes a tool to characterize and fit a phenomenological SCR model. The required data to fit the parameters in the SCR model can be obtained using an engine testbench in less than 3 days. The fit procedure is successfully implemented in a dedicated fit tool for the SCR model. This reduces the fit process of the model to a few days. The fitted model accurately predicts NO, NO2 and NH3 emissions for a wide range of emission cycles (see Figure 10 and Table 5). The NOx conversion of the SCR catalyst can be predicted within 10 % accuracy for a wide range of cycles (see Table 5). The model and fit tool are successfully used for SCR control and OBD development trajectories.

REFERENCES

1. Nebergall, Hagen and Owen, Selective catalytic reduction on-board diagnostics: past and future challenges, SAE 2005-01-3603, 2005

2. Schär, Onder, Elsener and Geering, Model-based

control of an SCR system for a mobile application, presented at SAE World Congress, 2004-05-0412, 2004

3. Chi and DaCosta, Modeling and control of a urea-

SCR aftertreatment system, presented at SAE World Congress, 2005-01-0966, 2005

4. Van Helden, Verbeek, Willems and Van der Welle,

Optimization of Urea SCR deNOx Systems for HD Diesel Engines, presented at SAE World Congress, 2004-01-0154, 2004

5. Winkler, Flörchinger, Patil, Gieshof, Spurk and

Pfeifer, Modeling of SCR DeNOx Catalyst – Looking at the Impact of Substrate Attributes, presented at SAE World Congress, 2003-01-0845, 2003

6. Van den Eijnden, E.A.C. and Cloudt, R “Calibration

method and apparatus for SCR catalyst systems”, European patent application, No. 07108151.7, May 14, 2007

7. Willems, Cloudt, Van den Eijnden, Van Genderen and Verbeek, De Jager, Boomsma, Is closed-loop SCR control required to meet future emission targets?, presented at SAE World Congress, 2007-01-1574, 2007

8. Wang, Yao, Shost, Yoo, Cabush, Racine, Cloudt and

Willems, Ammonia Sensor for Closed-Loop SCR Control, presented at SAE World Congress, 2008-01-0919, 2008

9. Song and Zhu, Model-based Closed-loop Control of

Urea SCR Exhaust Aftertreatment System for Diesel Engine, presented at SAE World Congress, 2002-01-0287, 2002

10. Hammer and Bröer, Plasma enhanced selective catalytic reduction of NOx for diesel cars, presented at SAE World Congress, 982428, 1998

CONTACT

Edwin van den Eijnden, M.Sc. Senior Project Engineer TNO Science and Industry Automotive Steenovenweg 1 P.O. Box 756 5700 AT Helmond The Netherlands +31 15 2697329

DEFINITIONS, ACRONYMS, ABBREVIATIONS

α catalytic-specific storage parameter

Ω NH3 storage capacity

µ viscosity

3NHθ NH3 surface coverage (i.e. number of actual

occupied sites divided by maximal number of active sites)

*

3NHθ critical NH3 surface coverage

ρ density

ε catalyst porosity

∆Hr reaction enthalpy

λ thermal conductivity

a specific catalyst area

C concentration

cp heat capacity

dh hydraulic diameter

E activation energy

0

ak adsorption rate constant

0

dk desorption rate constant

0

NOk Pre-exponential coefficient standard reaction

p pressure

R gas constant

hF friction coëfficient for flow through a channel

η dynamic viscosity

T temperature

v gas velocity

x location in catalyst

R universal gas constant

SV Space Velocity (residence time in catalyst)

SUBSCRIPTS:

a ambient

g gas phase

s solid phase

no NO emission component

n2 N2 emission component

ox oxidation

cat solid phase catalyst

APPENDIX

A brief description is given of the SCR catalyst model with only the NO reaction. The studied catalyst monolith is axially divided in N cells. In each cell, the mass and energy balances are solved simultaneously.

Mass balances

The mass balances for NH3 and NO are given.

- Solid phase: NH3 surface coverage θNH3 depends on

the adsorption and desorption of NH3 and the reaction of NH3 with NO:

Equation A 1

NOda

NHrrr

t−−=

∂

∂3

θ

- Gas phase: concentrations of NH3 and NO in the

catalyst channels are given by:

Equation A 2

)(33

da

NHNHrr

x

Cv

t

C−Ω−

∂

∂−=

∂

∂

Equation A 3

NONONO rx

Cv

t

CΩ−

∂

∂−=

∂

∂

where Ω represents the storage capacity of NH3 in the catalyst (in mol/m

3) and v is the gas velocity. In both

cases, this is the result of mass transport by convection and mass transfer between the gas and substrate.

The adsorption rate of NH3 on the catalyst is described by the following relation:

Equation A 4

)1(33

0

NHNHaa Ckr θ−=

The NH3 desorption model is based on the Temkin mechanism:

Equation A 5

3

3

0 )1(

0

NH

RT

E

dds

NHd

ekr θαθ−−

=

This rate describes the desorption from the catalyst surface, diffusion from the pores and diffusion to the gas flow. Note that the desorption rate also depends on the catalyst temperature Ts. Finally, the reaction rate of NO is linear to NO concentration and adsorbed NH3.

However, above the critical NH3 coverage *θ NH3 this

reaction rate is independent of the amount of adsorbed NH3:

Equation A 6

−=

−−*

3

3

31*0 NH

NH

s

NO

eCekr NHNO

RT

E

NONO

θ

θ

θ

Energy balances

For the gas phase, convective heat transport and heat transfer between the catalyst and the gas plays an important role:

Equation A 7

)(,, sgsgsg

g

gpgg

g

gpgg TTahx

Tcv

t

Tc −−

∂

∂−=

∂

∂↔↔ρερε

The energy balance of the solid phase consists of contributions due to conduction, heat transfer between gas and catalyst, heat transfer to the surrounding, and energy production by chemical reaction, respectively:

Equation A 8

rNOasasas

sgsgsgs

sgs

spsg

HrTTah

TTahx

T

t

Tc

∆Ω+−−

−+∂

∂−=

∂

∂−

↔↔

↔↔

)(

)()1()1(2

2

, λερε

Pressure drop

The pressure drop across the catalyst is determined from:

Equation A 9

2

)(

h

h

d

xvF

x

p ⋅⋅−=

∂∂ η