Calibration Exhaust-Aftertreatment Integrated, DoE-based · Exhaust-Aftertreatment Integrated, DoE-based Calibration 2012-01-1303 Published ... with DPF and SCR can also be used to

ABSTRACTFor on- and off highway applications in 2012 / 2014 newlegislative emissions requirements will be applied for bothEuropean (EURO 6 / stage 4) and US (US 2010 / Tier4 final)standards. Specifically the NOX-emission limit will belowered down to 0.46 g/kWh (net power > 56 kW (EU) /130kW (US) - 560 kW). While for the previous emissionslegislation various ways could be used to stay within theemissions limits (engine internal and aftertreatmentmeasures), DeNOX-aftertreatment systems will be mandatoryto reach future limits.

In these kinds of applications fuel consumption of the enginesis a very decisive selling argument for customers. Total costof ownership needs to be as low as possible. The trade-offbetween fuel consumption and NOX-emissions forcesmanufacturers to find an optimal solution, especially withregard to increasing fuel prices.

In state of the art calibration processes the aftertreatmentsystem is considered separately from the calibration of thethermodynamics. The thermodynamic engineers find the bestfuel consumption in steady state engine operating points andmost efficient combustion with regard to engine-outemissions which might be converted by an aftertreatment-system with an assumed conversion rate.

The problem associated with this approach is that thetransient and therefore the heat-up behavior of theaftertreatment system are not being considered. The heat-upbehavior becomes more and more important in particular fortest procedures including a cold started cycle such as WHTC

or NRTC. To overcome these problems a second mode forthe heating of the exhaust aftertreatment system (EATS) andsometimes a third mode with low NOX emissions will becalibrated separately. The optimization of the operationstrategy with all modes is mostly done at the test bench. Theoptimization is always done in a serial way when using thisapproach. This does not consider all calibration parameters atthe same time. Therefore, the overall fuel consumption or thetotal cost of ownership does not reach the optimum.

A new approach developed by FEV GmbH and the Institutefor Combustion Engines RWTH Aachen University (VKA)takes into account the aftertreatment system and all enginecalibration parameters from the first calibration step. VKAand FEV have developed a tool (SimEx) which is capable ofsimulating a freely configurable aftertreatment system. Theintegrated global engine DoE and an automatic optimizer willcalibrate all engine and EATS parameters. The target of theoptimization is an optimal solution for fuel consumption /total cost of ownership with regard to the transient tail-pipeemissions. The engineering target for the emissions in thehomologation cycle is typically the constraint. Along with thereduction of fuel consumption in the certification cycles thefuel consumption of the specific application can also be takeninto account during the calibration process through the use ofcustomer defined cycles. The total cost of ownership can beeffectively minimized through this process.

The parameters for optimization can be chosen completelyfreely e.g. injection and air path parameters for differentengine modes, operation strategy, EATS parameters like sizeand position. Furthermore different models can be included,

Exhaust-Aftertreatment Integrated, DoE-basedCalibration

2012-01-1303Published

04/16/2012

Yves Rosefort and Andreas WiartallaFEV GmbH

Henry Kwee and Stefan PischingerInstitute for Combustion Engines, RWTH

Copyright © 2012 SAE International

doi:10.4271/2012-01-1303

Gratis copy for Yves RosefortCopyright 2012 SAE International

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http://dx.doi.org/10.4271/2012-01-1303

such as ECU models for the operation strategy, catalyst agingmodels, and newly configured ones.

If use of the tool is begun in a state where the exhaustaftertreatment is not fixed, there is the opportunity to designand optimize the aftertreatment system for the lowest totalcost of ownership including the prices for precious metal andconsumed fuel over life-time.

Overall, this tool allows the integrated optimization of thewhole calibration process instead of calibrations based onindividually optimized parts.

INTRODUCTIONThe next step in the legislated emissions regulations formedium and heavy duty vehicles, as well as for buses, isplanned for 2012 with EURO VI1. Here the nitrogen oxideemissions (NOX) are further reduced from 2.0 g/kWh to 0.46g/kWh. As a result of this, On Board Diagnosis (OBD) limitvalues are also changed. While in EURO V the malfunctionindicator light (MIL) announces an error at 3.5 g/kWh andstarting from a value of 7 g/kWh the available output powerof the engine is reduced down to 60-75%, in EURO VI alimit value of 1.5 g/kWh is intended.

Furthermore, the certification cycle for commercial vehiclesis changed. The previously used European Transient Cycle(ETC) is replaced by the World Harmonized Transient Cycle(WHTC). Therefore, a certain level of development isneeded, since the mean temperature of the exhaust system ofthe WHTC is lower. The most important changes aresummarized again in table 1. Also, the emissions limits forindustrial engines for the US emissions legislation Tier4i andTier4f can be found in table 1 for engines higher 160 kWengine power. The emissions limits for Euro V and Tier 4i aswell as Euro VI and Tier 4f are quite similar, but use different

cycles with different speed, torque and temperaturecharacteristics.

A similar adjustment of the emissions regulations will also beobserved with industrial engines. The interest in fuelconsumption reduction is getting stronger at the same time.To fulfill the emissions targets with optimized fuelconsumption or minimized total cost of ownership, differentoptions can be used:

• Reduced engine out emissions [4]

◦ Optimized combustion system

◦ Optimized / higher boost pressure

◦ Higher injection pressure

◦ Increased EGR-Rates / efficient EGR cooling

◦ Efficient charge air cooling

• Exhaust aftertreatment

◦ DOC size and coating

◦ SCR size, type, and coating

◦ DPF size, substrate, and coating

◦ Exhaust gas aftertreatment system architecture

• Calibration

◦ Operation strategy (number of engine modes, andswitching strategy)

◦ Engine calibration in all engine modes

• ECU structure and sensors

◦ EATS control structure

1So far the limit values for EURO VI are not finally fixed. It is assumed that the limit values are reduced as described. This comes out from prior publications of the European Union of22.11.2010.

Table 1. Changes of the exhaust legislation concerning the EUROV/VI and tier4i/Tier4f standards:



◦ Combustion control structure

◦ Sensor layoutTo optimize the overall application all options which can bevaried have to be taken into account. The overall optimumcan not be found just by the variation of the individualparameters. All parameters have to be optimized in parallel.Due to the high number of parameters this is not possiblethrough engine testing.

To allow this complex optimization, a simulation programwas developed by FEV GmbH in co-operation with theInstitute for Combustion Engines RWTH Aachen University.This program, SimEx, is able to optimize all previouslymentioned and additional parameters as well as simulate theexhaust aftertreatment system, waste heat recovery andhybridization.

CERTIFICATIONThe certification is typically the constraint of everyoptimization. As an example, the certification according toEURO VI has a NOX limitation of 0.46 g/kWh and a PMlimitation of 10 mg/kWh. Because of the deviations of thesensors and the actuators with regards to the engine itself andthe exhaust aftertreatment system as well as aging of thecatalyst a NOX engineering target of 0.33 g/kWh is used forthe following considerations. In Figure 1 normalized enginespeed and engine torque of the WHTC cycle is shown. Besidethe transient test also the emission limits of the steady statetest (WHSC) and the Not To Exceed (NTE) area has to befulfilled.

Figure 1. Normalized engine speed and engine torque inWHTC

COMPLETE SYTEMThe complete system consists of an engine, including itscalibration, the exhaust aftertreatment system, as well as thecontroller concept. To simulate the complete system, the

following parts are included in the program SimEx. Theengine including the calibration can be simulated through thecapability to implement any global Simulink based DoEmodels from the test bench. This model is capable ofpredicting the emissions, the temperatures, as well as thebrake specific fuel consumption (BSFC). Further parameterscan also be included if necessary. The input parameters of themodel (named indirect parameter in Figure 2) are calibrationvalues like rail pressure, EGR rate, and injection timings andquantities. These maps can be smoothed by an algorithm.These smoothed input maps are used by the global models tocalculate the output values (direct parameters in Figure 2).These output values contain the emissions, exhaust massflow, temperatures, and also the BSFC. These outputs mapscan also be smoothed. Another very important outputparameter could also be the costs. The cost models should beset up with specific values to be able to calculate the totalcost of ownership. Therefore, the total engine life time has tobe defined. Examples for costs are system cost for the engine(life time related), system costs for EATS system (life timerelated), fuel costs (consumption related), as well as Adblue(urea) costs (consumption related). Some costs also should beset up as a function of other parameters. For example, withcatalysts the price is also a function of catalyst size to a largeextent. When the cost system is set up completely the totalcost of ownership can be optimized instead of fuelconsumption.

Figure 2. Overview SimEx optimizer

These map based models are combined with a turbo chargermodel which is used for transient cycles. Also transientcorrections are possible with regard to the emissions. As asecond input, catalyst data from a synthetic gas test bench orengine test bench is used. These models predict theconversion efficiency of the different catalyst/EATS typeslike DOC, SCR, LNT and DPF. The third part is the softwarestructure of the ECU with the strategy for the exhaustaftertreatment system (EATS) and the engine operationmodes. For example the SCR dosing strategy (e.g. with orwithout drift correction and NH3 storage model) is quiteimportant for the real EATS efficiency.



The complete system including all parts is shown in Figure 3.

Figure 3. System setup in SimEx

The optimization itself is completely flexible. The enginecalibration with all varied input values can be optimized, aswell as catalyst sizes, positions, and coatings. Different ECUstrategies can also be taken into account.

As an example, the Tier4 final emissions limit can be reachedby a particulate oxidation catalyst (POC) in combination witha high efficiency SCR system, or also a combustion systemwith low PM emissions in combination with such an SCRsystem (Figure 4). Tier4f emission limits can also be fulfilledby low- NOX combustion system in combination with a DPFonly. Last but not least a state of the art combustion systemwith DPF and SCR can also be used to fulfill the Tier4frequirements [6]. To be able to find the best way, allpossibilities have to be taken into account, with optimizedengine calibrations and also an adopted operation strategy.The simulation software SimEx has the ability to take intoaccount the different combustion systems as well as differentEATS concepts and define the best calibration and operationstrategy for each concept.

Figure 4. Different ways to reach emission limits

In Figure 5 an example of an optimization cycle is shown.The optimization starts with an initialization of the enginemaps and all other calibration parameters (like catalystcoatings and sizes) by the SimEx optimizer. This calibration

will be checked during the step “Model Evaluation” to ensurethat the calibration limits are not exceeded.

If the “Model Evaluation” is verified a conditioning WHTCwill be simulated. This has to be done, because theconversion efficiency during the cold cycle is stronglydependent on the NH3 start loading. This NH3 storagedepends on the cycle temperature, especially at the end, andalso the dosing strategy and ECU functionality. With regardto EU6 emissions legislation, two transient cycles have to beconducted with an intermediate soak time of 10 minutes forcool down. The weighting of the emissions results of thesecycles is 14% for the cold cycle and 86% for the warm cycle.The weighted cold and warm WHTC are used as a constraintfor NOX and PM emissions during the optimization. Forexample the NOX emission has to be lower than theengineering target of 0.33 g/kWh and the PM emission has tobe lower than the engineering target of e.g. 20 mg/kWhupstream DPF. If the constraint is fulfilled two customercycles, which represent the real operation condition, will besimulated. They can be weighted e.g. by 50% with respect tothe fuel consumption. This fuel consumption is the overalloptimization target during the optimization. Due to the fact,that the OBD limits also have to be fulfilled during thecustomer cycles this will be checked as a constraint duringthe optimization run.

Figure 5. Example for optimization run

OPTIMIZED VALUESDuring the optimization the combustion system, the enginecalibration, the EATS system as well as the ECUfunctionality can be optimized.

In Figure 6 shows different strategies to reduce the raw NOXemissions. With regard to the combustion system, the EGRcan be increased (1) to lower the NOX emissions and the railpressure can be increased (2) to lower the engine out PMemissions. Therefore the capability of peak cylinder pressurehas to be increased. If the EGR is increased further (3) therelative air to fuel ratio has to be reduced to lower the peak



cylinder pressure. By reduction of the compression ratio (ε)the peak cylinder pressure can be reduced further. [1]

Figure 6. Optimization combustion system in C100operation point [1]

Also highly efficient boosting [7] and cooling systems, incombination with higher EGR rates, can be applied to get abetter NOX / PM ratio.

To take different combustion systems into account, modelsfrom all relevant systems can be implemented and usedduring the optimization process.

In addition to the combustion system, the engine calibrationitself can also be optimized during the overall optimizationprocess. The basis to set up the DoE models is typically aglobal DoE (Design of Experiment), which also contains theengine speed and the torque as variation parameters in themodeling. The program is capable of handling nine engineoperation modes, which can be switched between, dependingon the ECU capabilities. Therefore, the operation strategy ofthe ECU can be implemented in the simulation tool. Anexample for the operation strategy could be the following(Figure 7):

• A heating mode (mode 1), whereby the exhaust gastemperature is maximized, in order to bring the exhaustsystem up to operating temperature, where the NH3-storagelevel of the SCR is filled sufficiently, and no urea dosage isstill permitted due to low temperature.

• A NOX optimized mode (mode 2) in order to avoid highNOX emissions when the SCR NH3-storage is almost empty,and

• A fuel consumption optimized operating mode (mode 3) forthe part of the cycle, in which the urea dosage is active andconversion rates of the SCR are high enough. This mode isalso of high importance with regard to customer use, becausetypical customer cycles have very often longer duration w/o

cold start and therefore sufficient EATS temperatures whichare needed for a high conversion efficiency.

Figure 7. Calibration based on three different modes ofoperation

To compare different ECU structures and capabilities, thestructure which should be compared can be implemented andthe optimizer can decide which one has the best capabilitywith regard to the overall optimization target.

In addition to the combustion system and engine calibrationthe EATS system also has to be optimized. With a suitableexhaust aftertreatment system setup consisting of DOC, DPFand SCR with suitable SCR technology with appropriate basecalibration (NH3- governor calibration, dosing temperaturethreshold), the operation strategy as well as the catalyst sizeand coating technology has to be defined (Figure 8).

Figure 8. Exhaust aftertreatment system setup

In addition to the catalyst size and the coating, the ECUsoftware capability is of major importance. Therefore this canalso be included in the simulation program SimEx (e.g. agingmodels, NH3 governors…). Also for these models the



comparison of different structures is also possible withSimEx.

Figure 9 shows examples of different possibilities for theengine operation strategies. In the first example, the switchfrom the heating mode into the NOX optimized mode is doneat an SCR temperature of 150 °C and into the fuelconsumption optimized mode at 250 °C. Also, a direct switchfrom NOX optimized mode to BSFC optimized mode as wellas a direct switch from heating to BSFC optimized mode isshown.

Figure 9. Variation of the points of operating modeswitching on the basis of the SCR temperature

While the test procedure described in Figure 5 takes about100 minutes in real time (without engine shut-off of approx. 8h to cool down for the cold start cycle), the simulation needsonly 4 seconds. Thus the variations suggested in Figure 9take about 3 minutes simulation time.

OPTIMIZATION EXAMPLESIn the following figures some examples of the SimExcapabilities are shown, to show the principle possibilities ofthe program. The graphs are examples with only 2-dimensional variations. The overall optimization is capable ofhandling any higher dimension. The number of variablesshould of course be minimized to the needed values tominimize the optimization time and reduce the risk of asystem which has several local maxima or minima whichmake the identification of the global maximum and minimumdifficult.

The first example is a single mode strategy with a DoEengine model and one engine maps set. The variationparameters are rail pressure, EGR rate and injection timing.The target is a Tier4f calibration with Cu-zeolite SCR andDPF. Therefore the optimization cycle is the following:

Figure 10. Optimization cycle Tier4f application

Overall, the fuel consumption for the customer cycle isreduced significantly during the optimization process (Figure11). The optimization is done in three generation steps whichinclude up to 6200 simulation cycles. The results of theplotted generations include only calibrations which fulfill the“Model Evaluation” and are within the emissions limits. Alsothe final engine maps for NOX, BSFC and Temperatureupstream turbine are shown in (Figure 11).

Figure 11. Reduced BSFC by optimization; final enginecalibration



The effects of catalyst size Figure 12 and NH3 loading andoperation strategy Figure 13 are shown as sensitivity studyinstead of an optimization to visualize the optimizationbackground. During the optimization the best combinationwill be found automatically as for the example in Figure 11.

The second example demonstrates the effect of the DOC sizeon the overall fuel consumption. A larger DOC size results ina higher NO2/NOX ratio and therefore in higher SCRefficiency [8]. The SCR efficiency depends on the NO2/NOXratio, especially in the lower temperature range. Thesimulation was done with two fixed engine map sets. Oneoperation mode is for heating, and the other for optimizedfuel consumption. Depending on the DOC size, the switchingtemperature between heating mode and BSFC optimizedoperation mode can be changed in order to achieve a constantNOX tailpipe emission (in this example NOX target = 0.3 g/kWh). At higher DOC volume the BSFC optimized mode canbe released earlier and therefore the overall fuel consumptionis reduced. (Figure 12). With further increased DOC size theheat mode is needed for a longer duration due to increasedtemperature losses and higher heat capacity.

Figure 12. Fuel consumption as a function of DOC size

The effect of the initial NH3 loading on the SCR catalyst issimulated as a third factor (Figure 13). The greater the initialNH3 load the earlier the BSFC optimized engine mode can beused. Therefore, the overall fuel consumption during theNRTC cycle can be lowered with an increased NH3 startingload. The start loading of a cycle is depending on theconditioning cycle, and in particular on the dosing strategy ofthe ECU. With higher NH3 load the risk of NH3 slip due tohigh temperature gradients over the cycle increases.Therefore, the overall dosing algorithm has to be quitecomplex in order to ensure high NH3 loading at a low NH3slip level [3], [5], [9].

Figure 13. Variation of NH3 loading of SCR

The ECU strategy could also help to reduce the fuelconsumption over a lifetime through use of a catalyst agingmodel. The aging of a DOC reduces the NO2/NOX ratio atlower temperatures. Due to that effect, a longer engineheating phase is necessary (Figure 14). Without an agingmodel the operation strategy has to be optimized for the worstcase (aged DOC). With an aging model the heat mode can beactivated for shorter times for a fresh catalyst and longer foraged ones based on the results from the simulation process.This helps to reduce fuel consumption over lifetime.

Figure 14. Operation strategy as a function of catalystaging

SUMMARY/CONCLUSIONSIn state of the art calibration processes the aftertreatmentsystem is considered separately from the calibration of thethermodynamics. The thermodynamic engineers define thebest fuel consumption in steady state engine operating pointsand most efficient combustion with regard of engine outemissions which might be converted by an aftertreatment-system with an assumed conversion rate. This could lead toproblems because the real EATS efficiency could be muchlower due to low exhaust gas temperatures at the EATSsystem in transient or cold start conditions.

A new approach based on the combined optimization ofdifferent parameters of the combustion system, engine



calibration, exhaust aftertreatment, as well as ECU functionsshows potential to further optimize the fuel consumptionwhile fulfilling the legislative emission demands. In additionto the optimization of fuel consumption this program furtherprovides the possibility for optimization of the total costs ofownership, which are typically the most important value forthe end customer.

The potential of this new approach has been demonstrated byseveral examples. It was e.g. pointed out that fuelconsumption might be reduced in the legislative test cycles aswell customer relevant cycles taking higher hardware costs(e.g. for the oxidation catalyst) into account. Furthermore theimportance of initial ammonia loading on the SCR catalystand thereby also the SCR control strategy (here especially theammonia storage control) as well as the potential of anadapted calibration over catalyst ageing with regard to fuelconsumption was shown.

To reduce the development time the program is able tooptimize all of the mentioned parameters in a simple waywith very short simulation times. For example one NRTCcycle can be calculated within one second using a standardPC.

REFERENCES1. Rosefort, Y., Körfer, T., Ruhkamp, L., Wiartalla, A.,Kinoo, B., Koehler, E., Tatur, M., Tomazic, D.:Verbrauchsoptimierte zukünftige Emissionskonzepte für HDOn- und Off-Road Motoren - innermotorischeEmissionsreduktion oder Abgas- nachbehandlung ?, 5.Internationales Forum Abgas- und Partikelemissionen,Ludwigsburg, 2010

2. Kwee, H., Rosefort, Y., Wiartalla, A.:Abgasnachbehandlung bei hybridisierten Nutzfahrzeug- undOffroadanwendungen, 9. FAD-Konferenz, Dresden/Germany, 03.-04.11.2011

3. Holderbaum, B. “Dosierstrategie für ein SCR-System zurNOX-Reduktion im Diesel-Pkw” Dissertation, RWTHAachen, 2009

4. Ruhkamp, L., Körfer, T., Wiartalla, .A., Neitz, M.,Tomazic, D., Tatur, M. : Post 2014 HD Diesel Engine

Concepts - Upcoming Demands and Innovative Approaches,5. Internationale MTZ Fachtagung Heavy-Duty-, On- undOff-Highway-Motoren Mannheim, 23.-24.11.2010

5. Schär, C., Onder, C., Geering, H., and Elsener, M.,“Control-Oriented Model of an SCR Catalytic ConverterSystem,” SAE Technical Paper 2004-01-0153, 2004, doi:10.4271/2004-01-0153.

6. Broll, P., Schraml, S., ZukünftigeAbgasnachbehandlungssysteme für Off-Road-Anwendungen,8. Dresdner Motorenkolloquium, 2009

7. Bülte, H., Schiffgens, H., Broll, P., Beberdick, W.,Technologiekonzept von DEUTZ zur Einhaltung derGrenzwerte der Abgasstufe US EPA Tier 4 und EU Stufe IVfür Motoren in Mobilen Arbeitsmaschinen, 17. AachenerKolloquium Fahrzeug- und Motorentechnik 2008

8. Müller, W., Lappas, I., Geißelmann, A.,Abgasnachbehandlungssysteme für Nutzfahrzeuge in On-und NonRoad-Anwendungen, 30. Internationales WienerMotorensymposium, 2009

9. Willems, F., Cloudt, R., Experimental Demonstration of aNew Model-Based SCR Control Strategy for Cleaner Heavy-Duty Diesel Engines, IEEE Transactions on Control SystemsTechnology, Vol. 19, No. 5, 2011

CONTACT INFORMATIONDr.-Ing. Yves [email protected]+49 241 5689 9900

The Engineering Meetings Board has approved this paper for publication. It hassuccessfully completed SAE's peer review process under the supervision of the sessionorganizer. This process requires a minimum of three (3) reviews by industry experts.

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Calibration Exhaust-Aftertreatment Integrated, DoE-based · Exhaust-Aftertreatment Integrated, DoE-based Calibration 2012-01-1303 Published ... with DPF and SCR can also be used to