Week-7 : Converting a detailed engine model to a FRM model

Aim:  To understand the concept of converting High-Fidelity Engine Model to Fast Running Model(FRM) and also building FRM Model using FRM builder approach.

Objective:

• To understand the concept of converting Engine Model to Fast Running Model(FRM) from Tutorial-9.
• Building FRM Model for following configuration using FRM builder approach 

                  > Bore 102 mm stroke 115 mm CR 17

                  > No of cylinder 6

                  > CI engine

                  > Twin Scroll Turbine

                  > GT Controller

Description:

The Fast Running Engine Models are designed in such a way that the simulations run faster. Generally we use detail modeling for capturing the wave dynamics that occurs in the intake and exhaust system. In order to capture those waves we need to have a very detailed model to capture the physics. Where as if the speed of the simulation running is necessary there we use use Fast Running Model. In GT-Power we follow two approaches i.e. Mean Value and FRM model. When the simulation speed is the main criteria the detailed model can be simplified into FRM's using the standard conversion process. The FRM Models are fully physical and dynamic in nature and also designed to run fast.  The mean-value approach is no feasible as we cannot capture the incylinder pressures and also it requires more data and statistics is involved. Where as in fast running approach we can retain the physical approach for the model and also get good accuracy. The FRM's are able to achieve such fast run times by two means i.e.(a)Increasing the simulation time step size (b)Decreasing the number of calculations per time step.

In FRM model there are two approaches i.e. FRM Converter & FRM Builder.The FRM converter helps to convert the detailed model to FRM, and in FRM builder the FRM is built from scratch using FRM Tool.

A. Exploring Tutorial-9 (FAST RUNNING MODEL):

Go to Tutorials > click on Modeling_Applications > go to Engine_Performance > select "09-FastRunningModel" > It consists of 8 steps which clearly show how the detailed model is converted into FastRunningModel.

In this model a 4-Cylinder Turbocharged Gasoline Direct Injection Engine. The turbine wastegate is being dynamically controlled in order to target the BMEP. A semi-predictive charge air cooler is included such that temperature is predicted as a function of inlet gas temperature and mass flow rate , coolant temperature and the charge air cooler effectiveness.

Detailed Model:

Initially launch the FRM Tags. 

In the FRM Tags the complete model is tagged i.e. it will show which parts can be combined and hence reduce the number of engine parts. The individual subsystem and the parts will get hihlighted on the map.

Now launch the FRM Converter. It is a tool that will help to track the FRM Conversion proces, organize the moell files automatically within subdirectories and provide result comparisions of key RLT's throught the conversion process. These generated files can be saved seperately and can be viewed at any time.

Now navigate to Tools > click on 'FRM Converter'. It will direct wheree we can create an FRM Conversion Project. Now this will validate the prevviously tagged subsystem and dislays finally if the validation is correct or not.

Click on Next if the Validation is sucessful. Then 'Accuracy Checking'  dialog box is opened. Set the tolerances as required and we can see the comparision plots will be generated and we will be warned if the values exceed the mentioned tolerence . After defining the tolerances click on Next to start the conversion process.

From now we should start simplifying the subsystems of the model. It is always better to start simplifying the subsystemthat is restricting the time step. For the basic models it is always better to start with exhaust manifold as here we have higher velocities which may restrict the actual time-step.

Step-1 Exhaust Manifold:

On clicking "Next(Begin First Step)" it wil direct to Model Reduction step-1a. Now in the Tagged Subsystems list select the "Exhaust Manifold 1".

Now after selecting the tagged Subsystem,click on Collapse to minimize the Model Reduction Window. So, from the top toolbar click on RunSetup. Now select the FlowControl option from the RunSetup window, then double tap on 'ExplicitControl'. Then 'Template:FlowControlExplicit' window opens, here change the value of Maximum Time Step to 720 and click Apply.

Now click on Expand to maximize the Model Reduction Window. Here we have two main parameters i.e. Simplify for Accuracy and Simplify for Speed. If accuracy is our main constraint then we select  Simplify for Accuracy option where as if our main constraint is speed then we select the  Simplify for Speed option.

In the same Model Reduction window if we click on 'Simplify Manually' this will alow to simplify the existing geometry for parts that wer not tagged or where a different combination of volumes is desired.

The discretization length in the pipe parts are overwritten with an automatically created parameter called "[dxe_FRM]". Hence the value is set to 300mm.

Similarly a parameter for heat transfer multiplier is used to calibrate the heat transfer in the combined volume to assure the turbine inlet temperature is maintained.

In order to verify the created parameters, open case setupand check the specified values. Here the heat transfer multiplier parameter is calibrated to match the turbine inlet temperature and hence the basic value of 1 is provided as reference.

Now go back to Model Reduction window and click Next (select Calibration). Here the simplified model is preprocessed in order to check if it is ready to run before setting up calibration.

Click on 'Configure Calibration' to open the Design Optimizer settings. 

Use the value selector and navigate to Turbine in RLT Selector and select Mass flow Averaged Inlet Temperature. The values of this parameter are taken automatically from the results of the baseline i.e. actual model. Hence this sees that the calibration will match the results of the baseline values or results.

Next, in the Design Optimizer go into "Factors" tab to define the factor along the lower and upper values for the Range and also the Resolution.

 Next, in the Designer Optimizer window select 'RUN CALIBRATION'. The simulations start running , once it is done go to results i.e click on Next.

In this Results page, click o RUN MODEL to run the complete set of cases using the optmized value.

Once the simulationof the model is complete close the simulation window. Next in the same Resuts Window click on "Populate Accuracy RLT Tables Below" in order to visualize and compare the results.

Here we can check the model results and chek if they are acceptable or not when compared to baseline values. After checking the values just click on 'Next (Begin New Step).

Step-2 Exhaust Pipes:

In this step, the exhaust pipes located near the turbine are simplified i.e. it is tagged as Exhaust Pipes-1 in the list. Now select the Exhaust Pipes-1 from the list and click on 'Simplify Accuracy' then click Yes to proceed. The pipe at the outlet of the turbine is combined with catalyst volumes into a single FlowSplit part. Now select the parts as shown and right click and click on Combine flow volume Wizards.

After opening the Combine Flow Volumes Wizard, select Combine flow volumes to flowsplit and click Next.

Click NEXT untill " Object Attribute Definition & Object Naming" window is seen. Once it is open double click on Flow split general and select 'EP1_Pdrop1-2'. Here set the object name to CAT and click Finish. Next close the Combine Flow Volumes Wizard.

 Once all the topoligies are converted, Open up the "CAT-1" part and go to the Thermal tab. Select the radio button for "Imposed Wall Temperature". Edit (double Click) the "GasT" Reference Object. Use the value selector for the "Wireless Signal or RLT (X)" object value and replace "tav:catconn1" with "tav0:CAT-1” and click "OK". Close the object dialog for the "CAT-1".

Now we need to Open Case Setup and set the value of [DIA_Exhaust_Pipes_1_Step_2] to 40 mm. This is only an initial guess, and will be used to calibrate the Turbine Outlet Pressure. In the FRM Converter, click "Next (Set Calibration)".

In the Calibration page, select Case 1 for calibration. Under "Configure calibration…", set the Response RLT to the Turbine Massflow-Averaged Outlet Pressure (trb-poa:Turbine), and set the lower and upper values of the range of DIA_Exhaust_Pipes_1_Step_2 at 35 and 45 mm, respectively.

Run the calibration, and once it is finished, click "Next". In the Results page, click "Run Model" to rerun the full set of Cases. After checking model results and assuring that they are satisfactory, click "Next (Begin Next Step)".

Step 3: Intake Manifold
Name the step "Intake Manifold". Select Intake Manifold 1 from the Tagged Subsystems list. Click on "Simplify for Accuracy". Click "Yes" on the Delete Part dialog; this will be handled later. Collapse the FRM Converter to provide space to work in the model map.

Step 4: Compressor Outlet Pipes
Name the step "Compressor Outlet Pipes". Select Boost Pipes 1 from the Tagged Subsystems list. Click on "Simplify for Accuracy". Collapse the FRM Converter to provide space to work in the model map.
Since the intercooler core (previously a bundle of pipes) has been replaced with a FlowSplit and "HeatExchangerConn" part. The "BP1_CAC1-2 HeatExchangerConn" part will be used to assure the intercooler outlet temperature is maintained. Double-click on the "BP1_CAC1-2 HeatExchangerConn" part and set the Imposed Fluid Temperature to 300 (this will be overridden), then click OK.

Step 5: Intake Pipes
Name the step "Intake Pipes". Select Intake Pipes 1 from the Tagged Subsystems list. Click on "Simplify for Accuracy". Collapse the FRM Converter to provide space to work in the model map.Go into Case Setup and set the value of [DIA_Intake_Pipes_1_Step_5] to 45 mm (this is just an initial guess).
Finally, Expand the FRM Converter and click "Next (Set Calibration)" to preprocess the model and continue to Calibration.

Step 6: Additional Changes
Name the step "Additional Changes". Select Exhaust Manifold 1 from the Tagged Subsystems list. Click on "Simplify for Speed". 

Run the calibration, and once it is finished, click "Next".
In the Results page, click "Run Model" to rerun the full set of Cases. Check the model results and assure that they are satisfactory. The model should now be running around 1 xRT. In the FRM Converter, click "Next (Begin Next Step)".

Step 7: Cylinder Slaving 

The final simplification to the model will utilize the Cylinder Slaving capability of the EngCylinder component. Name the new step "Cylinder Slaving", then collapse the FRM Converter to provide space to work in the model map.

For the full cylinder slaving to function, each Valve must be specified as either an Intake or Exhaust valve. Find the ValveCamConn template in the model tree on the left and select "Edit Parts in Spreadsheet" view. The intake and exhaust valves are easy to distinguish. The intake ones start with "IM1_PortRun” and the exhaust ones start with "EM1_PostRun". Go to the Output folder and set "Valve Type for RLT Variable Calculations" to "intake", respectively "exhaust".

 Running an FRM

 On setting up all cases we can now extract the frm model.

B. Building FRM Model using FRM builder approach:

Specifications:

• Bore 102 mm stroke 115 mm CR 17
• No of cylinder 6
• CI engine
• Twin Scroll Turbine

Setting Up the Model:-

Step-1

Step-2

Step-3

Step-4

Step-5

Step-6

 FRM Model:

 Result:

 Cases: 1-5

Cases: 6-10

Cases: 11-15

Cases: 16-20

Cases: 21-25

• All the above cases are solved in a very short time as we have considered FRM model for the simulation.
• In the initial cases it is observed that the BSFC value is higher.This might be higher beecause of low Air/Fuel ratio.
• Generally for diesel engine it should not be operated at a very low air/fuel ratios, always it should be on higher values.
• It is also observed that on increasing the Air-Fuel ratio the value of BSFC started decreasing.
• Higher the air available, the soot formation is completely low.

Conclusion:

• The concept of converting Engine Model to Fast Running Model(FRM) from Tutorial-9 is explored.
• Building FRM Model for specific configuration using FRM builder approach.