Aftertreatment Model for SI and CI Engine

AIM

• Explore -Examples provided below and write a detailed report
  1. Detailed SI engine+aftertreatment
  2. Detailed CI engine aftertreatment
• Compare results of all cases and give your comments

SI Engine + AfterTreatment

• This model demonstrates modeling an SI engine combined with a detailed TWC in a secondary aftertreatment circuit. The base model Spark Ignition 4 cylinder with EGRController was combined with the after treatment.
• It is a port injection model with a fluid selected as indolene liq -NASA and injector will need a Part Name for the Air Mass Flow Rate Sensor.
• The cylinder geometry is shown below :

• Engine type, cylinder arrangement, firing order, and other engine block and crankshaft characteristics have been defined.
• For our cylinder, we have selected, SIWiebe combustion model WoschniGT heat transfer model

• And for monitoring exhaust gases we have selected the Nox emission model and CO emission model, which is shown below. The NO calculation is based on the extended Zeldovich mechanism.

• The base model consists of ERG(Exhaust Gas Recirculation controller) model, which is used to target the EGR rate by controlling the throttle angle or orifice diameter.

So the base model is shown below :

• And for the aftertreatment model Three way catalyst is used. A three-way catalyst oxidizes exhaust gas pollutants - both hydrocarbons (CmHn) and carbon monoxide (CO) - and reduces nitrogen oxides (NO_x) into the harmless components water (H₂O), nitrogen (N₂) and carbon dioxide (CO₂).
• In this model, two different exhaust is selected. The original exhaust system remains connected to the rest of the engine model. And for copied exhaust system An EndFlowInletSpecies part feeds the inlet boundary conditions to the QS circuit.
• Mass flow rate, temperature, and species concentrations are sensed from the original exhaust system, averaged over the cycle, and imposed on the QS circuit's EndFlowInletSpecies.
• This template describes boundary conditions of flow, temperature, and species composition.

• And the species sensed by it shown below :

• The `Composition_XYTable` should include all relevant species to the aftertreatment system. N2 should be set to "def" and all other species concentrations can be arbitrary entered because they will also be overridden by actuation. Other species that the cylinders emit, such as OH, N, and O, can be omitted because their concentrations are so small that they do not affect the overall mass of the exhaust gas.
• To begin, the modeler must first make assumptions about the composition of the hydrocarbons that the aftertreament system will see and use.
• Implementing fuel conversion in the model is fairly simple. From the explicit circuit, one would sense the mole fractions of vapor and liquid fuel, water vapor, and oxygen. The vapor and liquid fuel signals would be added to get a total fuel concentration. Then `MathEquation` would be used to calculate the mole fractions of propylene, propane, water vapor, and oxygen to be actuated in the QS circuit.
• The equation used for indolene is shown below, which is used to calculate the amount of hydrocarbon sensed by the QS circuit:

`C_7.93 H_14.8 + 1.0587H_2O rarr (2.6433)(0.8C_3H_6 + 0.2C_3H_8) + 0.5293O_2`

• A different `CatalystBrick` object has been used for each circuit. The "explicit" object has a discretization length that is similar to that used in the other exhaust system components and imposed wall temperature has been set for the thermal option.

• The "QS" object should have a discretization length around 2 - 5 mm to accommodate the Advanced Adaptive chemistry solver. Its thermal option should be set to "Calculated Wall Temperature".

• The surface reaction is defined in the surface reaction template. This template specifies catalytic surface reaction kinetics that requires the modeling of storage or coverage.

• The catalyst bricks and particulate filters in the QS circuit that solve chemistry will have more accurate thermal solutions than those in the explicit circuit. Therefore, the thermal solutions of the QS components should be transferred to the explicit components so that backpressure and wave dynamics are accurate.

The Aftertreatment part is shown below :

RUN SETUP

• The exhaust system is configured to use the QS flow solver with an appropriate time step. Improved Solution Sequence for Multi-Circuit Models checkbox. With this checkbox activated, in FlowControl there will be a new attribute visible to assign a different Time Control Flag to different circuits.
  
• The explicit circuit (column 1) will have a "periodic" time control flag and use the explicit flow solver. The QS circuit (column 2) will have a "continuous" time control flag and use the QS flow solver.

• The Time Step and Solution Control Object is mostly used to define the time step for each circuit. For explicit time step remain as "def" and for QS time step is 0.1 sec.

CASE SETUP

The given solution run at three different case setup :

RESULT

TABLE SUMMARY

• The given table shows the simulation summary of our data for different Air-fuel ratio.
• We can see as we increase the air-fuel ratio, Brake Power, and torque decrease for constant RPM.

CYLINDER DATA

• The given table shows the cylinder output data for different air-fuel ratios. We can see for the SI engine lambda value is less than 1.
• We get maximum pressure in the cylinder around 51.34 bar.
• Because of the use of the aftertreatment model the NOx emission is around 14.7 g/KW-h and for hydrocarbon it is 0.97 g/KW-h for our model.

CI Engine + AfterTreatment

• This model demonstrates an integrated model with a detailed engine and detailed aftertreatment system running one steady-state operating point. The detailed engine model comes from the Diesel_Wastegate controller.
• The detailed aftertreatment system is a combination of the DOC_Sampara_and_Bissett and SCR_Fe_Zeolite example models along with a DPF setup for passive regeneration only, and the same urea injection system used in the EngineState_Vehicle_Aftertreatment.

• It is a Fast Running Model(FRM) with a turbine and compressor map model connected by a shaft and also a wastegate controller to control the boot pressure requirement.

• Engine type, cylinder arrangement, firing order, and other engine block and crankshaft characteristics have been defined.
• For our cylinder, we have selected the Direct-Injection Diesel Wiebe Combustion Model, and the WoschiniGT heat transfer model.

• It is a direct-injection model and soot emission object map is selected.

• The base model consists of ERG(Exhaust Gas Recirculation controller) model, which is used to target the EGR rate by controlling the throttle angle or orifice diameter.

• And for aftertreatment model DOC(Diesel Oxidation Catalyst) with SCR(Selective Catalyst Reduction ) and the particulate filter is used
• In this model, two different exhaust is selected. The original exhaust system remains connected to the rest of the engine model. And for copied exhaust system An EndFlowInletSpecies part feeds the inlet boundary conditions to the QS circuit.
• Mass flow rate, temperature, and species concentrations are sensed from the original exhaust system, averaged over the cycle, and imposed on the QS circuit's EndFlowInletSpecies.
• The fuel converter converts unburned fuel emitted from the cylinders to representative hydrocarbons used in the aftertreatment reaction mechanisms. The converter works by considering a balanced reaction of fuel and excess water vapor and converting to the smaller hydrocarbons with a small amount of generated oxygen.
• This template describes boundary conditions of flow, temperature, and species composition.

• And the species sensed by it are :

• To begin, the modeler must first make assumptions about the composition of the hydrocarbons that the aftertreament system will see and use. The following example considers a diesel fuel (`C_13.5H_23.6`) as the engine fuel and assumes the unburned fuel gets converted to 50% fast-oxidizing propylene (`C_3H_6`), 25% slower-oxidizing toluene (`C_7H_8`), and 25% dodecane (`C_12H_26`).
• Implementing fuel conversion in the model is fairly simple. From the explicit circuit, one would sense the mole fractions of vapor and liquid fuel, water vapor, and oxygen. The vapor and liquid fuel signals would be added to get a total fuel concentration. Then `MathEquation` would be used to calculate the mole fractions of propylene, propane, water vapor, and oxygen to be actuated in the QS circuit.
• The equation used for this model is shown below, which is used to calculate the number of species sensed by the QS circuit:

`C_13.5H_23.6 + 0.5350H_2O rarr (2.25 C_3H_6 + 0.4821C_7H_8 + 0.2813C_12H_26) + 0.2675O_2`

• A different `CatalystBrick` object has been used for each circuit. The "explicit" object has a discretization length that is similar to that used in the other exhaust system components and imposed wall temperature has been set for the thermal option. After that, it passes through the DPF. This template is for a wall-flow filter (Diesel Particulate Filter, Gasoline Particulate Filter) after that it passes through the SCR.
• The "QS" object should have a discretization length around 2 - 5 mm to accommodate the Advanced Adaptive chemistry solver. Its thermal option should be set to "Calculated Wall Temperature".

• The surface reaction is defined in the surface reaction template. This template specifies catalytic surface reaction kinetics that requires the modeling of storage or coverage.

• After that, it passes through the Diesel particulate filter. The filter models three physical processes of a wall-flow particulate filter:
  • pressure drop
  • particulate matter (soot) and ash filtration
  • soot regeneration and catalytic reactions
• For the Substrate Thermal Properties Object use Value Selector to search the Template/Object library for the pre-defined 'MaterialThermalProp' object named "Cordierite" from the Library. This object represents a typical catalyst ceramic substrate material with a density based on the porosity of 0.35.

• The Modeling Options folder contains additional attributes for more detailed DPF modeling. The Filtration Object is used to define the particulate matter filtration and pressure drop due to loading. The Regeneration Object defines DPF regeneration chemistry, and the 2D/3D Object attribute allows the ability to model 2D and 3D effects. Filtration and regeneration will be covered later in the tutorial.

• Substrate reaction part name is the surface reaction template which is linked to it:

• After that air enters the SCR template. For the Substrate Thermal Properties Object use Value Selector to search the Template/Object library for the pre-defined 'MaterialThermalProp' object named "Cordierite" from the Library. This object represents typical catalyst ceramic substrate material with density based on the porosity of 0.35.
• The urea solution Is injected into a flow split pipe before SCR. The injected Fluid Object can be set to the predefined fluid object in the Template Library called "ureasolution-NASA", which represents a fluid with composition. After the urea solution is admitted to the system, the breakdown of ammonia can be handled in a couple of different ways.

• Liquid water evaporates, leaving solid urea particles, (NH2)2CO, entrained in the gas phase (note that urea and water will not be in a 1:1 molar ratio in the liquid/solution phase).
• Solid urea undergoes thermolysis as it decomposes into ammonia (NH3) and iso-cyanic acid (HNCO).
• HNCO subsequently undergoes hydrolysis to form ammonia and carbon dioxide.
• The reaction occurring here is shown below :

• Site element for the zeolite selected is zeolite and the chemical reaction occurring in SCR are :

RUN SETUP

• The exhaust system is configured to use the QS flow solver with an appropriate time step. Improved Solution Sequence for Multi-Circuit Models checkbox. With this checkbox activated, in FlowControl there will be a new attribute visible to assign a different Time Control Flag to different circuits.

• The explicit circuit (column 1) will have a "periodic" time control flag and use the explicit flow solver. The QS circuit (column 2) will have a "continuous" time control flag and use the QS flow solver.

CASE SETUP

The given solution run at this case setup :

RESULT

Table Summary

• The given table the simulation summary for our model at RPM 2000. For a particular RPM, we can see that the power output is 66.6 HP and torque is 236.6 N-m.

Cylinder Summary

• The given table shows the cylinder output for our model.
• We can see that the lambda value is greater than 1.
• Emission output for NOx is 6.78 g/KW-h and for hydrocarbon, it is 0.11 g/KW-h.

OBSERVATION

• We can see differences in the modeling of aftertreatment. For the SI engine, it is a two-way catalyst and for CI engine it is DOC(Diesel Oxidation Catalyst)+ DPF(Diesel Particle Filter) +SCR(Selective Oxidation Catalyst).
• Also, the emission quantities for a diesel engine are quite less as compared to the SI engine.
• In SI engine Exhaust Gas Recirculation(EGR) is used and for the Diesel engine, Wastegate Controller is used is our case setup.
• We can see that lambda for the SI engine is less than 1 and for diesel engines is greater than 1.

Conclusion

• The given simulation for SI engine with Aftreatment and Diesel engine with Aftertreatment has been successfully performed.
• The difference between both models has been successfully noted in terms of aftertreatment and engine output.