SCRTrainning1

1、SCR Controls Training 1System Structure, Components, and parametersBaohua 05/2007About This Training Audience: CPE engineers and ATI engineers. Goal: understanding SCR basics, knowledge of SCR control structure, parameters, and functions.Contents Catalyst basics and SCR basics Critical parameters in

2、 SCR control System configuration SCR controls overview, functions, and modules Introduction to tuning FF and Limit methods Tradeoffs and levers Efficiency Calculation, Limit Control, Hydrolysis Compensation, Feed-forward Control, Urea Dosing Command Generation and Correction in SCR control Catalyst

3、 BasicsOverall time for reaction = Bulk diff. time + Pore diff. time + Catalytic reaction time反应时间越短，效率越高。反应时间越短，效率越高。SCR BasicsLow temperature (450C) are not desirable fordosing ureaCritical ParametersCritical Parameters: 1. SCR temperature2. Exhaust flow rate/Space velocity3. NH3 concentration (Do

4、sing command)4. SCR inlet NOx concentration5. NO2/NO ratioNOx Conversion Efficiency At Given NO2/NO Split RatioRed: DeNH3 efficiencyBlack: DeNOx efficiencyAt given NO2/NO split ratioDeNH3 efficiency = ( DeNOx efficiency * NH3-NOx reaction ratio) / ANRNOx Conversion Efficiency Effects NO2/NO Split Ra

5、tioEffects NO2/NO split ratioSystem ConfigurationDOCDPFSCRSCRUrea injectorDelta PT1T2T3NOx sensorAMOXT5NOx sensor Required by EBUUrea Dosing PumpECMVehicle Harness (OEM)Urea TankAir SolenoidT4PNOx Sensor electronicsOnly for US marketSignal Path in SCR ControlsCES Data VectorSCR Control SystemControl

6、 Module Structure Functional StructureH:fes_controlswwSCRCore2releasep210000BuildFilesmdlSimulink modules can be found in:SCR Control Blocks Overview- SCPSCP: generate values for parameters used in SCR controlsSCR Control Blocks Overview- SCRMax. DeNOx Eff. is calculated in this blockSCR Control Blo

7、cks Overview- SCLDosing Limits: 1. SCR Temperature; 2. Exh. Flow; 3. Dosing Cmd. Inc. Rate; 4. Max./Min. Allowed Dosing Cmd., 5. Engine Status, 6. Doser StatusSCR Control Blocks Overview- SCDSCR Control TuningThe system tuning is for parametersused for optimizing the deNOx systemperformance.The SCR

8、control tuning is forparameters determined by SCR catalyst, doser, and system configuration. Tradeoffs in SCR System Tuning 1. Dosing economy and deNOx performance; 2. System capability and dosing economy; 3. NOx sensitivity and efficiency sensitivity.SystemLevers 1. The Weighing Factor (a Weighing

9、Factor table and a feed-back algorithm will be available in Aug. release); 2. NOx target table (Aug. release); 3. NH3 slip limit table (Aug. release); 4. Direct dosing command table.Feed-forward method and Limit MethodFF Method: Dosing cmd. is generated to make the NOx level at SCR outlet lower than

10、 required value (NOx Target).Limit Method: The max. dosing cmd. limited to the max. allowed NH3 slip is calculated.Final Command = Limit Cmd. + (1 - weighingfacotr)*FF Cmd.Max. urea dosing cmd.limited to the max. NH3 slip allowed by EPA/EuroUsing min (Required Eff., Max. Eff.) for calculating the NE

11、EDED urea dosing cmd.System Capability, Dosing Economy, and DeNOx PerformanceSystem is capable:Catalyst is big,engine_out NOx is low,exh. flow is low,exh. T is high,System is less capable:Catalyst is small,engine_out NOx is high,exh. flow is high,exh. T is low,When system becomes less capable, well

12、havesmaller weighing factor changing range.Sensitivity TradeoffLimit cmd. is NOT sensitive to NOx level, while sensitive to deNH3 eff.FF cmd. is sensitive to NOx level, while INSENSITIVE to deNOx eff.(Example)SCR ControlDeNOx Efficiency CalculationdeNOx Efficiency for a Given Exh. Flow/Space Velocit

13、yEfficiency Calculation FlowchartControl ModulesLimit Dosing Command CalculationLimit Dosing Cmd. Calculation Block Diagram“Limit” Urea Dosing Command Calculation Basic Equations)1/(slipiCCNOxNHSlipiCCC3When NH3/NOx 1, (1) When NH3/NOx NH3 + HNCO (step1)HNCO +H2O - NH3 + CO2 (step2) The hydrolysis p

14、rocess CAN NOT be accomplished in exhaust air. At 330C, only 20% urea is decomposed (thermolysis, step1). Even at 400C, only 50% urea is decomposed (thermolysis ,step1). The complete hydrolysis of urea needs the help of catalyst, therefore, the effective catalyst for SCR reaction is decreased. In ca

15、lculating urea dosing command, we need compensate for the effects of hydrolysis.Hydrolysis ProcessEffects of the hydrolysis effectively shortens the SCR length lHydrolysis Compensation ModulesFeed-forward Dosing Command CalculationFeed-forward Control Block DiagramBasic Equation Used in Calculating

16、Needed Dosing Cmd. (FF Cmd.)(3) / )(NOxslipNOxiCCC: NH3 to NOx ratio in reaction, changes with NO2/NO;CNOx: NOx concentration at inlet;: deNOx efficiency when ANR equals .Ci: NH3 concentration at inlet (Dosing cmd. is calculated using Ci);CslipNOx: Max. allowed NOx slip (NOx target) at tailpipe;Feed

17、-forward CalculationFeed-forward Control Modules (1) Capability Calculation-1Feed-forward Control Modules (1) Capability Calculation-2Feed-forward Control Modules (2) Feed-forward Calculation-1Feed-forward Control Modules (2) Feed-forward Calculation-2When System is NOT capable, FF cmd is CLAMPED to

18、 Limit cmd.SCR Dosing Command GenerationControl Modules - 1Control Modules - 2Dosing Command CorrectionControl Modules - 1Control Modules - 2Summary-1 1. The control system basically include a feed-forward control that is used for calculating the necessary dosing command and a limit control that is for generating the max. allowed dosing command. 2. In tuning the system, we have three tradeoffs: dosing economy and performance; system capability and dosing economy; NOx sensitivity and efficiency sensitivity. 3. Leve