diesel engine scr control: current development and future challenges · 2017-08-29 · 122 emiss....

Emiss. Control Sci. Technol. (2015) 1:121–133DOI 10.1007/s40825-015-0013-z

REVIEW

Diesel Engine SCR Control: Current Developmentand Future Challenges

Xinmei Yuan1 ·Hongqi Liu1 ·Ying Gao1

Received: 27 December 2014 / Accepted: 6 April 2015 / Published online: 9 May 2015© Springer SIP, AG 2015

Abstract To meet increasingly stringent emission legis-lations, selective catalytic reduction (SCR) becomes anessential aftertreatment technique. This paper presents acomprehensive overview of the current development andfuture challenges of SCR control technologies. In this paper,SCR system hardware configurations are introduced, mod-eling and calibration of the SCR converter are provided,and current control strategies are summarized and reviewed.Finally, primary areas driving the future of SCR controlresearch are discussed.

Keywords Diesel engines · Selective catalytic reduction(SCR) · Modeling · Control · Ammonia storage

1 Introduction

With a growing concern for environmental problems, engineemission regulations are becoming increasingly stringent.Using European heavy duty emissions regulations as anexample, as shown in Fig. 1, the NOx emission limits havebeen reduced from 8.0 to 0.46 g/kWh with a much strictdriving cycle—the world harmonized test cycle (WHTC).Additionally, the NH3 slip concentration is limited to10 ppm in Euro VI for the first time. Therefore, selective

� Xinmei [email protected]

1 State Key Lab. of Automotive Simulation and Control,Jilin University, 130025, Changchun,Jilin, People’s Republic of China

catalytic reduction (SCR) becomes an essential aftertreat-ment technique. In the past 10 years, extensive researchhas been conducted, leading to improved NOx conversionefficiency and NH3 slip prevention.

This paper presents a comprehensive overview of thecurrent development and future challenges of SCR controlsystems. The commonly used SCR hardware configurationis introduced. The modeling and calibration of the SCRsystem is then described. Different control strategies arecovered. To understand the road ahead, the primary chal-lenges of SCR control are addressed, and conclusions arethen summarized.

2 System Configuration

The control system’s performance is limited by the system’shardware configuration, which includes the plant, actua-tors, and sensors. A typical SCR system layout is shown inFig. 2.

The SCR converter consists of substrates on which a cat-alytic coating has been deposited. Among the SCR catalyticformulations, V/W/TiO2 and metal exchange zeolites arethe most widely used. V/W/TiO2 is tolerant to sulfur poi-soning [22] and the metal (Cu/Fe) exchange zeolites havebetter low temperature performance and thermal durabil-ity [49]. In Fig. 2, the diesel oxidation catalyst (DOC) andthe ammonia oxidation catalyst (AMOX) are optional. TheDOC is primarily intended to burn off the residual hydrocar-bons and CO in the exhausts, but also can be used to changethe exhaust gas temperature and to promote the oxidation ofNO to NO2, improving the NOx reduction efficiency [29,35]. The downstream AMOX is used to oxidize excess NH3

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122 Emiss. Control Sci. Technol. (2015) 1:121–133

Fig. 1 NOx limits of Europeanheavy duty emissionsregulations

[10, 35]. With the AMOX downstream, the NOx conversionefficiency and the NH3 slip control are decoupled and onlythe NOx conversion efficiency needs to be considered in thecontrol design. The disadvantage of the AMOX is that theoxidized NH3 is wasted and it may lead to the production ofunwanted N2O and NOx [47].

The dosing valve is the primary actuator that implementsthe SCR control strategy. The reductant agent is injectedinto the exhaust gas by the dosing valve in response tothe controller’s command and decomposes to NH3 for thereduction reaction. The installation position and character-istic of the dosing valve are important for the control ofammonia [13].

The sensors up- and downstream measure the tempera-ture and the concentrations of NOx and NH3. These sensorsmay be installed between the SCR and the AMOX whenpossible [31]. Though the cost of exhaust gas (NOx or NH3)sensors is high, these sensors are necessary for SCR closed-loop control. However, current commercially available NOx

sensors are significantly sensitive to NH3 (also referred to as

Fig. 2 Typical SCR system layout

the cross-coupling effect), which limits their use in feedbackcontrols. An excitation filter [39, 42] and an extendedKalman filter [20] are used to estimate the real NOx concen-tration. However, the bandwidth of these types of filters maylimit their performance in transient conditions. Ammoniasensors have become available in recent years, which offermany advantages [12, 46] compared with the controls basedstrictly on the NOx sensor’s feedback. In addition, thesenew controls have superior performance in transient condi-tions without the cross-coupling issue [54]. Currently, thereare studies focusing on applications using information froma combination of NOx and NH3 sensors, further improv-ing system performance. The downstream NOx sensor isrequired by on-board diagnostics (OBD) regulations.

3 Modeling and Calibration

A high fidelity SCR model capable of correctly predict-ing the gaseous concentrations and internal states underboth steady-state and transient conditions is valuable fordeveloping control strategies and diagnostic functions [49].Although there is no universally accepted chemical reactionmechanism for SCR catalysts, the Eley-Rideal mechanismis widely adopted and has been demonstrated to agree withexperimental data over a wide range of operating condi-tions [36, 42, 47]. Based on the Eley-Rideal mechanism,the modeling of the SCR converter is reviewed in thissection.

3.1 Gas-Solid Catalyst Reaction Mechanism

First, the 32.5 % urea solution is injected into the exhaustgas, where the water evaporates. The (NH2)2CO is then

Emiss. Control Sci. Technol. (2015) 1:121–133 123

Table 1 Primary reactions inSCR converter Reaction Equation Eqution no.

Pyrolysis reaction (NH2)2CO−→ HNCO + NH3 (1)

Hydrolysis reaction HNCO+H2O −→ NH3+ CO2 (2)

Adsorption/desorption NH3 ←→ NH∗3 (3)/(7)

Standard SCR reaction 4NH∗3+ 4NO −→ 4N2+ 6H2O (4)

Fast SCR reaction 4NH∗3+ 2NO + 2NO2 −→ 4N2+ 6H2O (5)

Slow SCR reaction 8NH∗3+ 6NO2 −→ 7N2+ 12H2O (6)

Oxidization reaction 4NH∗3+ 3O2 −→ 2N2 + 6H2O (8)

decomposed thermally into HNCO and NH3 in accordancewith the reaction below.

(NH2)2CO −→ HNCO + NH3 (1)

Icocyanic acid also forms NH3 with the hydrolysis reaction.

HNCO + H2O −→ NH3 + CO2 (2)

According to a majority of the studies [29, 42, 54], theeffect of the urea decomposition above on SCR catalyst per-formance can be assumed to be negligible when the exhausttemperature is greater than 200–250 ◦C. When consideringthe dynamic of urea decomposition, the related modelingdescriptions can be found in [7, 37], and additional detailsof the decomposition mechanism are found in [24].

Then, the formed NH3 is adsorbed by the SCR catalyst,and the adsorption reaction is expressed as

NH3 −→ NH∗3 (3)

With the adsorbed, gaseous NOx reductive reactions occur.Among these reactions, there are two predominant SCRreactions: the standard SCR reaction (4) and the fast SCRreaction (5).

4NH∗3 + 4NO + 4O2 −→ 4N2 + 6H2O (4)

4NH∗3 + 2NO + 2NO2 −→ 4N2 + 6H2O (5)

Although the fast SCR reaction is much faster than thestandard SCR reaction, the ratio of NO to NO2 determineswhich reaction is primarily considered when modeling the

process. For a majority of heavy duty diesel engines, theamount of NO2 is much less than the NO in the origi-nal exhaust gas [24, 51]. However, with a DOC installedupstream, the ratio may be different [35]. The reductivereaction can also be with pure NO2 as shown in Eq. 6. Thisreaction is referred to as the slow SCR reaction (also called“NO2 SCR” reaction)

8NH∗3 + 6NO2 −→ 7N2 + 12H2O (6)

The adsorbed NH∗3 can also be desorbed or oxidized to

N2. The desorption reaction and the oxidization reaction canbe expressed as Eqs. 7 and 8, respectively.

NH∗3 −→ NH3 (7)

4NH∗3 + 3O2 −→ 2N2 + 6H2O (8)

It is also reported that there are two different ammoniastorage sites with both sites supporting NH3 adsorption anddesorption reactions. These dual-site modeling approachesachieve an accurate description of the ammonia adsorp-tion/desorption process in an extended temperature window(50–550 ◦C) [11, 49, 50].

The primary reactions in SCR converter is summarizedin Table 1 and illustrated in Fig. 3. Several other reactionsreferred in SCR converter for different studies are listed inTable 2. In general, reactions Eqs. 3 to 8 are widely used forSCR control, and other reactions are normally neglected but

Fig. 3 Illustration of theprimary chemical reactions inSCR converter


Table 2 Other reactions inSCR converter Equation Reference

4NH∗3+ 6NO2 −→ 5N2+ 6H2O [41]

20NH∗3+ 12NO +9O2 −→ 16N2+ 30H2O [29]

2NH∗3+ 3N2O −→ 4N2+ 3H2O [28]

4NH∗3+ 5O2 −→ 4NO + 6H2O [7, 9, 19, 24, 28, 29, 31, 54]

4NH∗3+ 4NO + 3O2 −→ 4N2O + 6H2O [41]

2NH∗3+ 2NO2 −→ N2+ N2O + 3H2O [3, 28, 29]

6NH∗3+ 8NO2 −→ 7N2O + 9H2O [49]

2NH∗3+ 2NO2 −→ NH4NO3+N2+ H2O [41]

NO + 0.5O2 −→ NO2 [16, 29, 46, 49]

may be considered depending on the catalyst formulationand the operating temperature range.

3.2 Control Oriented Modeling of the SCR Converter

In terms of the gas flow and heat transfer inside the SCRconverter, the partial differential equations (PDEs) shouldbe used to describe the convection and diffusion phenom-ena. The finite volume method (FVM) is adopted to solvethe PDEs by means of calculation domain discretizationalong the flow direction into n cells, as shown in Fig. 4. Atthe cell’s interface, the output variables of the ith cell areconsidered as the input of the (i + 1)th cell. All variablesare assumed to be homogenous in each cell. Therefore, with

regard to each cell, the original PDEs are transformed intoordinary differential equations (ODEs).

With each model serving specific purposes, the numberof cells may be different. Examples of cells used in ref-erences are shown in Table 3. It is clear that the numberof cells is a trade-off between model fidelity and execu-tion time. Therefore, high cell-count models are used forthe simulation plant while low cell-count models are bettersuited to real-time control applications.

Neglecting the influence of the energy exotherm orendotherm by the chemical reactions [41, 56], the ther-mal dynamic is decoupled and can be modeled separately.The energy balance of the converter solid phase consistsof contributions primarily due to heat convection between

Fig. 4 Discretization of theSCR converter


Table 3 Number of cells for different modeling purpose

Reference No. of cells Purpose

[29]2 Control

24 Plant

[41]2 Control

3 Plant

[31]2 (1 for ARC∗) Control

6 (2 for ARC∗) Plant

[34]1 Control

15 Plant

[61]

1 Control

1 Plant (scenario 1)

4 Plant (scenario 2)

[8] 2 Control

[17] 5 Plant

[49] 10 Control

[54] 12 Plant

[11] 31 Not specified

∗ARC is the SCR downstream cell serves as an ammoniareservoir cell [31]

the gas and the catalyst, while other studies consider heatradiation to the surroundings. A typical description of thetemperature state equation in one cell is shown below [41]

dT

dt= Kc(Ti − T ) − Ka(T

4in − T 4

a ) (9)

where Ti is the input exhaust gas temperature, T is the tem-perature of the cell, and Ta is the ambient temperature. Kc isa lumped parameter related to thermal convection, and Ka

is a lumped parameter related to converter-to-ambient heatradiation.

Following Arrhenius’ equation, the general chemicalreaction rate constant expression is defined as

kj (T ) = AjeEjRT (10)

where subscript j represents the corresponding reaction, k isthe reaction rate constant, A is the pre-exponential factor, Eis the activation energy, and R is the universal gas constant.The rate equation is given as

rj (T ) = kj (T )CαEC

βF (11)

where r is the reaction rate, CE and CF are the concen-trations of the reagents, α and β are the coefficients of thechemical reactions [62]. Because (11) is an element reactionequation in a complex reaction process, the overall reac-tion rate expression may vary. This is why chemical reactionexpressions are in the same forms as Eqs. 10 and 11, butmay vary slightly in details.

Applying the mass conservation law, the reaction rate isused and the nonlinear state equations of ammonia coverageratio, the concentrations of gas phase NOx (or NO, NO2)and NH3 in one cell can be expressed as

d

dt

⎡⎣

θ

CNOx

CNH3

⎤⎦ = f (θ, CNOx , CNH3 , kj (T ), d1, d2, . . .) (12)

where θ is ammonia surface coverage ratio, C is the concen-tration, and d1, d2, . . . are the system inputs, including cellinlet NOx and NH3 concentration, exhaust gas mass flow,etc. As an example, a commonly used mass balance equa-tion set of control oriented SCR model is shown below [42,58].

⎧⎪⎨⎪⎩

dθdt = 1

cs

(kAds(1 − θ)CNH3 − kDesθ − kRedθCNOx − kOxi

)dCNOx

dt= a0vf lowCin

NOx− (

a0vf low + kRedθ)CNOx

dCNH3dt

= a0vf lowCinNH3

+ kDesθ − (a0vf low + kAds(1 − θ)

)CNH3

(13)

where cs is the concentration of active surface atoms withrespect to gas volume in converter, kAds, kDes, kRed, and kOxiare the reaction rate constants of the adsorption, desorp-tion, reduction, and oxidization, respectively, vf low is thegas space velocity, CNOin

xis the inlet NOx concentration of

the cell, and CNHin3is the inlet NH3 concentration of the cell.

a0 is defined as

a0 = n

εCVC

where n is the number of cells, εC is the ratio of gasto total converter volume, and VC is the volume of cat-alytic converter. In order to be used in real-time application,only dominate dynamics of the system are concerned incontrol oriented modeling. Therefore, the impact of otherprocesses, such as the gas-solid mass and heat transfer, andof intraporous diffusion, are not detailed modeled in Eq. 13,

From Eqs. 9 and 13, it can be calculated that there willbe 4×n (5×n if NO and NO2 considered separately) differ-ential equations for an n-cell SCR model. Studies show thatthe time constants of reduction and oxidization reactionsare much smaller than those of adsorption and desorption.Therefore, the dynamics of gas concentrations are gener-ally neglected [8, 41, 54]. Furthermore, because the thermalmodel can be solved separately and there is no oscilla-tion in the system, it is also feasible to simply approximatethe chemical reaction dynamic as a first-order linear sys-tem with variable parameters [7, 22, 37, 38]. Therefore,these first-order dynamics represent the ammonia storagechanges.

Current SCR model for real-time application is lim-ited by the computing resources of the microcontroller.With the rapid development of microcontroller technol-ogy, more advanced discretization or solving methods are


expected to be used for the SCR PDEs, such as non-uniform meshing, adaptive meshing and characteristic linemethod.

3.3 Calibration of the SCR Model

Generally, a flow bench or engine test bench is used forSCR characterization measurements. On a flow test bench,conditions can be manually set up and optimally controlledto study individual SCR reactions under specific conditions[49]. However, the engine exhaust gas composition is com-plex and difficult to control, and neglected reactions mayinfluence the SCR reaction. Because of this, calibration onthe engine test bench provides more practical results forSCR engine applications [14].

Depending on the SCR reactions to be considered, differ-ent experiments are carried out to characterize correspond-ing subsets of model parameters. The normally adoptedcalibration is divided into steady-state and transient experi-ments.

Because thermal and ammonia adsorption/desorptiondynamics dominate the SCR reaction, the related parametersneed to be calibrated in a transient experiment. Step signalsare normally used to excite the plant transient performancein the entire frequency range. Therefore, approximate step-wise changes in inlet temperature or NH3 concentration are

applied in these experiments. Examples of these experimen-tal results are shown in Fig. 5. In these experimental results,thermal and ammonia adsorption/desorption dynamics bothshow approximate first-order responses.

The remaining chemical reaction-related parameters arecalibrated in a steady state condition. Different operatingconditions combined with typical exhaust gas tempera-tures and space velocities (or engine speeds) are selectedto cover the emission standard driving cycle. To char-acterize tens of the parameters in the different chemicalreactions and to avoid over fitting, different test protocolsmust be designed to separate each reaction in the tests.It is simpler to change the gas compositions and isolatekey reactions on a flow test bench. However, reasonableapproximations are effective on an engine test bench. Forexample, neglecting the oxidized NH3 at low temperaturesand neglecting the standard or fast reaction depending onthe ratio of NO to NO2 are reasonable approximations. Formore details about the calibration process, refer to [14, 49].The model parameters are obtained using a nonlinear least-squares method. The cost function to be minimized can beexpressed as:

J (p1, . . . , pi) =n∑

k=0

((yest (tk, p1, . . . , pi) − yexp (tk)

)2)

(14)

Fig. 5 Experimental results ofthermal and ammoniaadsorption/desorption responses


where p1, . . . , pi are the unknown parameters to be cal-ibrated, t0, . . . , tn are sampling times, and yest and yexp

are the model estimated and experimentally measured data,respectively.

The calibrated models are normally validated by a stan-dard driving cycle, such as European stationary cycle (ESC),World Harmonized Stationary Cycle (WHSC), Europeantransient cycle (ETC), and WHTC. Currently, the averagemodel prediction error is within 10 % [14], However, thepredictions of NH3 slip have much larger errors. This isbecause the absolute value of the NH3 slip is too small rel-ative to the NOx concentration and because the ammoniastorage estimate is difficult to validate. Therefore, NH3 slipestimation appears to be the most challenging task in SCRmodeling [53].

4 Control

As presented in the previous section, it is difficult to developan accurate SCR catalytic converter model for controlpurposes. One reason is that the gas flow in the con-verter is not homogeneous. Therefore, a fine mesh of the3D volume is needed for accurate calculations [56]. How-ever, this type of calculation is not feasible for a currentdosing control unit (DCU). Across the entire operating tem-perature range, there are multiple side reactions, only afraction of which can be considered in the model, particu-larly in a transient state. As a result, the dynamic model ofthe SCR converter is important yet insufficient for controlpurpose.

4.1 Control Goals

Because the NOx limits in the emission standards areaverage values, (0.46 g/kWh in WHTC for Euro VI), thelimits cannot be directly used in control. It is challeng-ing to control an average value in a real-time controllerbecause the future operating conditions are unpredictable.Therefore, NOx conversion efficiency or inefficiency isused as the control, as in [29, 42]. The NH3 slip limitis a real-time value (10 ppm in WHTC for Euro VI),yet critical because the “buffer” effect of the ammoniastorage may cause the NH3 slip uncontrollable [55]. Theammonia storage dominates the dynamics of the chem-ical reaction; therefore, it can be used to control NOx

and NH3 concentrations. As a result, a majority of recentstudies change the control goal in the SCR converter tothe ammonia coverage ratio. Because the ammonia stor-age varies spatially from the converter inlet to the outlet,the set-point values of the desired ammonia coverage ratiobecome difficult to determine. Average value approachesare primarily used [17, 18, 29, 54]. However, in [42],

simulation results show that NH3 slip is more closely relatedto the surface coverage ratio in the SCR cell downstream,and in [31], the desired set-point values up- and down-stream are given separately. Another issue for the ammoniastorage control is the consideration of the ammonia stor-age capacity. The ammonia storage capacity changes withtemperature. The NH3 storage capacity decreases rapidlyas the temperature rises. Therefore, unacceptably highNH3 slips may occur during sudden temperature increases[55]. With unpredictable temperature changes, the set-point value of the ammonia coverage ratio becomes verycritical.

4.2 Open-Loop Control

In open-loop control, lookup tables are developed to coverthe different factors in the system, and the final urea dosingquantity is calculated from these lookup tables, as shown inFig. 6a. Reference [43] gives a brief introduction to the nor-mal open-loop control strategy in a BOSCH aftertreatmentsystem. Similar approaches have been proven to be suffi-cient to meet Euro IV and Euro V emission standards, whichrequire approximately 50–60 and 70–80 % NOx reduc-tion, respectively [55]. Although this method is widely usedand suitable for a nonlinear system, it requires significantcalibration work and is inaccurate in transient states.

4.3 Direct Feedback Control

With the NOx sensor downstream, the information fromthe sensor can be used as feedback to guarantee theplant’s robustness against disturbances, as shown in Fig. 6b.Proportional-integral (PI) controllers are used for feed-back control [42, 48]. However, NOx sensor informa-tion by itself is insufficient for direct feedback con-trol in all case. For example, the NOx feedback sig-nals are similar when the urea dosing valve is partiallyblocked and when the catalyst is aging, even though therequirements for the feedback controlled urea dosage areopposed in these two situations. Moreover, consideringthe cross-sensitivity and time delay of the NOx sensor,and the low thermal and chemical bandwidth of the SCRsystem [29], direct feedback control is very challeng-ing for highly dynamic systems. Therefore, direct feed-back control is universally combined with feedforwardcontrollers.

4.4 Adaptive Control

Adaptive control is suitable for solving nonlinear and uncer-tainty problems in the SCR system. A typical adaptive con-trol structure is shown in Fig. 6c. An adaptive PI controlleris derived and experimentally tested in [38] to demonstrate


Fig. 6 SCR control structures


the capability to achieve conversion efficiencies of 90 %with ammonia slip under 10 ppm. Based on the NH3

sensor feedback, an adaptive learning algorithm for max-imizing the allowable ammonia storage value is proposedin [12, 54]. A model reference adaptive control (MRAC)system is introduced in [7]. In the MRAC system, a sim-ple first-order model is used as an SCR reference model,and an adaptation mechanism is designed for updating theadjustable parameters.

4.5 State Estimation

With increasing focus on the ammonia storage in SCR sys-tems, internal state estimation is becoming popular in SCRcontrol. A universal SCR control structure with state esti-mation is shown in Fig. 6d. Because the SCR properties arevery nonlinear, which affect computational efforts, a sim-ple “time constant model” is used to estimate the ammoniastorage state in [37], and a fourth-order linearized modelis used to design an estimator for CNO, CNO2 , CNH3, andθ . To enhance the robustness of the observers, a nonlin-ear observer (sliding mode observer) is designed to esti-mate the ammonia coverage ratio in [19]. The NOx sensorcross-coupling effect is one of the key issues in apply-ing closed-loop observer, an interpreted coverage value isused in an ammonia coverage ratio observer to solve thecross-coupling effect [4]. In [20, 30], Extended Kalmanfilter (EKF) is used to reject the cross-coupling effect andmeasurements noises.

4.6 Model Predictive Control

Model predictive control (MPC) is particularly beneficialin multi-input multi-output systems when more than onecontrol objective or constraint exist. The MPC structure isshown in Fig. 6e. In [8], NOx concentration, NH3 slip, andurea dosing quantity are included in the cost function. How-ever, in [29], NOx conversion inefficiency and urea dosingquantity are considered in the cost function and NH3 slipis set as a “soft constraint” for the average value control.NH3 slip is enforced only at the starting time and again atequilibrium. In order to realize the MPC in a real-time appli-cation, the algorithm must be simplified. Generally, only alinearized model can be used; the parameter control movesis small and the input is assumed to be constant beyond thecontrol horizon. Though it is reported that the receding hori-zon nature of MPC ensures that relatively little optimalityis lost by truncating the input horizon to a relatively smallsequence [29], the MPC control performance affected bylinearized model accuracy, a highly dynamic driving cycleand no consideration of the maximum allowable ammo-nia storage is still an interesting and open topic in thisfield.

5 Future Challenges

To address increasingly stringent emission standards in thefuture, there remain issues to address to further improvethe SCR system performance. Moreover, advances in mea-surement, materials, and control technology are expected tobring new research trends and industrial interest.

5.1 Low Temperature

For Euro VI legislation, a new test cycle WHTC with acold start procedure is introduced, indicating that low tem-perature system performance is attracting greater attentionin industry. The control of NOx conversion efficiency andammonia storage at low temperatures is very challenging.

The NOx conversion efficiency is limited by urea decom-position and catalyst activity. Typically, the effect of ureadecomposition in Eqs. 1 and 2 is slow and incompletebelow 200 ◦C, affecting NH3 concentration involved inthe reduction reaction, and the byproducts such as cya-nuric acid (C3H3N3O3), melamine, ammelide, and amme-line may be formed upstream of the SCR catalyst at lowtemperature [44], which could possibly clog the catalyst.Current catalysts for automobile applications are highlydependent on the temperature, particularly at tempera-tures below 250 ◦C, where the conversion rate declinesquickly [26]. For these reasons, majority of control strate-gies are bounded by a low temperature constraint, andthe urea dosing is deactivated below this temperature[29, 43].

The low-temperature issue is caused by the physicalbasis of the chemical reaction properties; therefore, thefundamental solution is to improve the system configura-tion. Solid SCR (SSCR) is introduced to avoid the ureadecomposition problem. Solid ammonium salt is used toproduce gas phase ammonia by thermal decompositioncontrol, which has been experimentally demonstrated toimprove the NOx reduction at low temperatures [22, 27].Catalytic materials need to be improved to assure highperformance in terms of de-NOx efficiency, selectivity,and cost. This implies development of new catalytic for-mulations with improved activity in the low-temperatureregion (T < 200 ◦C) and greater durability [11]. The chal-lenge for the control strategy at low temperatures is theammonia storage limitation. As referred to in the previoussection, with a very low NOx conversion efficiency andincreased ammonia storage capacity, the model of the sys-tem becomes an integrator with reduced damping, wheresmall errors can increase over time. Therefore, to avoid anunexpected NH3 slip caused by a sudden rise in temper-ature, the accuracy of the ammonia storage model or theammonia coverage ratio observer is more important for lowtemperature.


5.2 Coordinate Control

The tightened limits of PM and NOx in emission legisla-tions are difficult to satisfy with SCR or diesel particulatefilter (DPF)-only aftertreatment systems. The combinationof exhaust gas recycling (EGR), DPF, and SCR technologiesis widely adopted for Euro VI and US 2010 applications,bringing additional degrees of freedom to the control sys-tem. In these systems, the exhaust line layouts directly affectthe control considerations. Figure 7a–c shows three typicalconfigurations [15, 52].

Figure 7a is a widely proposed configuration. The DPFis placed immediately after the DOC. The urea dosage takesplace downstream of the DPF and upstream of the SCR cat-alyst. The DOC and DPF upstream are able to change theNO/NO2 ratio, and a perfect 1:1 ratio could enhance the fastSCR reaction in Eq. 5. An active NO/NO2 ratio control isproposed in [6], and the simulation results show significantimprovement to SCR performance. However, the simulationassumes that all states can be accurately measured. The DPFplacement in front of the SCR catalyst would be ideal forits regeneration management. However, this DPF placementwould poses significant problems for the SCR cold start effi-ciency due to the large DPF thermal mass [52]. Additionally,because the DPF regeneration temperature is up to 650 ◦C,a catalyst with high temperature durability must be selected[59].

Figure 7b describes a DOC followed by a connectingpipe where the urea is dosed. The SCR catalyst is thenplaced upstream of the DPF. In this case, the oxidization ofNO can be controlled by the DOC as previously referred.However, this process must also be carefully considered.It is reported that excess NO2 is produced by the DOCat temperatures between 300 and 400 ◦C, where a slowSCR reaction (6) replaces the fast SCR reaction (5) instead[45]. Placing the SCR catalyst upstream of the DPF reducesits warm-up time. However, the exhaust gas reaching theDPF will be poor in NO2, therefore minimizing the pas-sive regeneration potential and increasing the regenerationfrequency [52].

In Fig. 7c, the SCR is placed upstream of by the DOCand DPF. This concept offers some advantages. First, theSCR catalyst is not exposed to high soot regeneration tem-peratures. This allows the use of a wider selection of SCRcatalysts. Second, the DOC and the DPF in this config-uration will act as slip catalysts for the SCR system, butto prevent the production of unwanted N2O and NOx,the NH3 slip from SCR should also be strictly limited.Furthermore, positioning the SCR catalyst closest to theengine is an advantage during cold-start operation becausethe SCR catalyst heats quickly [15]. The disadvantagesinclude a high NO/NO2 ratio and reduced NO2 down-stream of the SCR, reducing the passive regeneration of theDOC.

Generally, the integration of different aftertreatmentdevices brings additional operating condition control witheach device. The primary challenges are the temperature andthe gas composition (the ratios of NO, NO2 and other gases)coordinate controls for the entire exhaust line. The tradeoffbetween the three configurations can potentially be solvedby integrating the SCR catalyst into the particulate filter asone multifunctional unit, as shown in Fig. 7d [1, 5, 21].Studies show that comparable NOx conversion efficiencycan be achieved by the integrated SCR on DPF catalyst toconventional flow-through catalysts [21]. However, the low-temperature NOx conversion efficiency, NO2/NOx ratiocontrol and sulfur poisoning problems are still the con-cerns for the integrated catalyst. Additionally, moving theSCR closer to the engine may expose it to the risk ofdeposits of particulate matter, especially at low tempera-ture, which is also a disadvantage of the configurations inFig. 7b, c.

5.3 Ammonia Storage Measurement

Ammonia storage is now considered to be unmeasurable,though it is the key factor that dominates the SCR reactiondynamics and NH3 slip. In order to estimate the ammoniastorage level, a chemical element balance is always used tocalculate the amount of ammonia remaining in the converter

Fig. 7 Schematic of typicallayouts for integration of theaftertreatment system


[37, 60]. The direct measurement of the current storage ofthe ammonia would be a much more reliable method.

The impedance of a film of catalyst material depends onthe amount of stored ammonia. During a short local temper-ature increase in which ammonia desorbs, the conductivityis measured. The conductivity change can be seen as ameasure for the ammonia loading of the catalyst [25, 32].

A contactless way of ammonia storage measurementhas been proposed. Because the zeolite material changesits electrical properties when the ammonia is stored inthe zeolites, electromagnetic waves are excited by probefeeds and the reflected signals are measured. Experimen-tal results show that distinct resonance frequencies dependnearly linearly on the ammonia loading of a zeolite-basedSCR catalyst. Therefore, the amount of stored ammonia canbe determined directly [33, 40].

Though electric properties providing a positive out-look for ammonia storage measurement, current workscan only be seen as a first attempt to show the possibil-ity of monitoring the ammonia loading. With respect to areal-world automotive application, important questions suchas the operating condition variations, aging effects, andcross-sensitivities must be considered [40].

Recently, the spatially resolved capillary inlet infraredspectroscopy (Spaci-IR) technique is used to study theammonia storage distribution and determining the ammoniastorage capacity in the SCR converter [50]. This techniqueis expected to greatly improve the modeling and calibrationof the SCR model.

5.4 Aging

The NOx reduction activity and hydrolyzing activitydecrease over long-term use, a phenomenon generallyreferred to as SCR catalyst aging. For an aging catalyst,emission limits become much more critical for control, andcatalyst degradation must be monitored by OBD accordingto current emission regulations.

There are several factors that affect SCR catalyst degra-dation, including hydrothermal aging, sulfur or phosphorouspoisoning, and injected urea-related deposits [23, 24, 26,57]. Though the nature of aging characteristics is com-plex and the process is time-consuming, there are a limitednumber of published studies available. Based experimentaldata, ammonia storage capacity is described as a functionof aging time and temperature in [2], which is expectedto be used as a feed-forward control algorithm to calcu-late the degradation of an SCR catalyst based on timeand temperature history. Because the degradation processis very slow, long-term estimation or adaptation can beused online. Ammonia storage capacity is considered to be

directly related to the SCR catalyst’s aging and an EKF isproposed to estimate this slow time-varying factor [30].

6 Conclusion

This paper has outlined an overview of SCR control sys-tems. From a control point of view, there are three primarytechnologies of concern: system configuration, modeling,and control strategy.

System configuration is the basis of the chemicalreaction. SCR catalyst materials, system layout, character-istics of the converter, sensors, and actuators all directlyaffect SCR control performance. Devices with increasedperformance at low temperatures and system layouts ofmulti-aftertreatment devices, including a DOC, DPF, andAMOX are popular topics.

The nature of chemical reactions in the SCR converteris very complex, nonlinear, and difficult to measure. Accu-rately modeling and understanding the states in the con-verter across a wide range of operating conditions is stillvery challenging. Direct measurement of ammonia stor-age may increase opportunities for significant improvement.Additionally, the modeling of aging catalysts is necessary.

Based on the physical model, the SCR control strategydesign is similar to other industry applications. Addition-ally, these points may need special attention: (1) the desiredcontrol values must be carefully considered because thedriving conditions in the future are unknown; (2) the domi-nant dynamics of the system are very slow and there are sig-nificant time delays in the feedback signals; (3) linearizationand parameter adaption should be considered because thesystem is very nonlinear. Nevertheless, as computer tech-nologies advance, more computationally-intensive high per-formance algorithms for this nonlinear system are expectedto be proposed.

Acknowledgments The authors would like to thank Yongjun Shufor assisting in preparation of Fig. 1.

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