Week-7 : Converting a detailed engine model to a FRM model
AIM
* To explore tutorial 9 which in details explains how to convert a existing model into a fast running model (FRM).
* To build a FRM using FRM builder approach with
bore = 102 mm
Stroke = 115 mm
Compression ratio (CR) = 17
CI engine, Twin Scroll Turbine, GT controller
INTRODUCTION
FRM is abbriviated form of Fast running model, as far as GT-suite models are concerned throughout we have run many models either it is pretaining to engine, aftertreatment or engine + aftertreatment, turbochargers etc, all these are detailed models. When simulation is run for different cases in any of the detailed model it takes considerable amount of time, being a detailed model there will be lot of components, for example if we take a intake manifold in a detailed engine model it is being modeled with number of circular pipes, orifices and flow splits, so when simulation is run the flow equations (Navier stokes equation) needs to be solved in each of these components and hence it becomes time consuming. To compensate for the time consumption Fast running models are developed, in a broad way we can put it like FRM sums up different components into one component with simplified geometry and attributes. If we take the same example of intake manifold, instead of circular pipes, orifices and flow splits in FRM it will be a single entity or a geometry called intake manifold.
When FRM is used instead of detailed model ofcourse we have an edge over the simulation time, CPU time but at the same time the accuracy of the results will be bit compromised also the fidelity of the model will be low. Purpose of building a FRM is that when software in loop, hardware in loop etc testing is being done there will be a real time ECU or any software plugged to the models, if detailed model is used it will be difficult to match up with the real time hardware or softwares so FRM comes handy.
Exploring tutorial number 9:
* The tutorial explains in detail how the a high fidelity (detailed model) can be converted into a FRM using the FRM converter tool. FRM converter is a handy tool provided by GT-suite which keeps track of the entire conversion process.
* First we need to select a detailed model so as to convert it into FRM, this tutorial number 9 considers the example case SI_4Cyl_GDI_Turbo_WOT, a snip of project map of this detailed model is as shown below.
* We can either convert this detailed model into FRM based on accuracy or speed, in this tutorial the simplification is done based on accuracy so as to compare the engine performance parameters from detailed model as well as FRM and to determine the accuracy.
* Detailed model which is open in GT-suite is first done FRM tagging, this option can be seen under the tools tab. It tags all the subsystems in the detailed model. We can start by simplifying the exhaust system, since high velocity flow is more at the exhaust this affects the courant number and hence simplification is started from the exhaust manifold.
* Before we apply simplifications to the exhaust manifold, the maximum time step limit of 1 crank angle degree will be removed. This is a default limit that is imposed when an Eng Cylinder component is in the model. If the courant condition alows for a larger time step to increase in size. When the exhaust system is simplified it looks as follow,
* Similar to exhaust system is exhasut from the turbine, a snip of the turbine exhaust system from detailed model is shown below,
* This system is simplified in same way as that of exhaust system of the engine. When simplified we get the exhaust from the turbine as,
This completes the second step in conversion of detailed model to FRM and hence we have the complete project map of the second step with simplified exhaust and turbine exhaust.
* Next is simplification of intake manifold, the steps followed are same as followed for exhaust system, minor modifications need to be made to the characteristic length andexpansion diameter attributes value for the combined flow.
* The characteristic length is defined as the distance that the flow entering the boundary will travel before crossing another boundary or impacting a surface. Boundary #1 represents the boundary between the IM1_pipe1-1. The fluid entering the flowsplit in this direction can travel all the way across the plenum before impacting a surface, so characteristic length is equal to the length of the plenum.
* The other boundaries are adjacent to the runners, where backflow from the cylinder would travel across the diameter of the plenum before impacting the plenum wall, which is why they are given a characteristic length of 70 mm.
* The expansion diameter defines the maximum diameter that the flow may expand to after entering a boundary. For boundary#1 it is simply the diameter of the plenum because the fluid expands from IM1_pipe1-1 into the plenum in a straight direction. The expansion diameters for the other boundaries are equal to the plenum diameter multiplied by 1.4. This is because as the fluid backflows into the plenum, it encounters a wider cross section than the plenum diameter to expand into. A snip of the modification window is given below,
This completes the step3 of the conversion, a snip of the same is given below.
* Next is simplification of compressor outlet pipes, select boost pipes 1 from the tagged subsystem list click on simplify by accuracy. Since the intercooler core has been replaced with a flowsplit and HeatExchangerConn part. The BP1_CACC1-2 HeatExchangerConn part will be used to assure the intercooler outlet temperature is maintained.
* When the compressor coutlet pipes are simplified this completed the step4 and hence we have
* Intake pipes select intake pipes 1 from the tagged subsystem list. Click on simplify for accuracy. Collapse the FRM converter to provide space to work in the model map.This completes the simpyfing process. We have somethings like below after step5
* We started with simplifying exhaust system on accuracy basis now we can further simplify the exhaust system on time step basis, by doing this we get a much simplified exhaust system as shown below,
* Final step or simplifying done is cylinder slaving. By right clicking on any of the EngCylinder slaving option. For full cylinder slaving to function, each valve must be specified as either an intake or exhaust valve.
So above tutorial gives us an idea of how the existing detailed model can be converted.
* To build a FRM:
Second part of this challenge is to build a FRM with following parameters,
bore = 102 mm
Stroke = 115 mm
Compression ratio (CR) = 17
CI engine, Twin Scroll Turbine, GT controller.
Main purpose of building a FRM is to check how much the simulation time as well as CPU time vary, so let us first take a detailed CI model which has a turbo and a controller.
Let us consider Diesel_6Cyl_TC example case, this model has a twin turbocharger, a control unit, 6 cylinders. This example resembles much to the FRM we are building.
Simulation is run at engine speed = 1500 rpm, below is the snip that shows the other parameters. Even this detailed 6 cylinder engine model is set with above mentioned bore, stroke and compression ratio values.
Following is the time taken for the simulation to run,
Now let us build a FRM and then run the simulaton,
Under tools option FRM builder option is available, when we click this option we get the below window where CI engine is selected.
* When we click next we get attributes that define bore, stroke and compression ratio. Below is the snip of the window that shows the attributes defined,
* When we click next we get a window where the type of turbo is set,
* By clicking next we define the number of cylinders,
* In last window the controller type is set,
This gives us a engine model as shown below,
Result obtained as follows,
* If we compare the time taken to complete the simulation in detailed model as well as FRM it is seen that the detailed model took less time and FRM took more time, but the more time taken by the FRM is because of number of periods for which the simulation is run. FRM is run for 128 periods and time taken for same is 10.24 sec. But detailed model run for 75 periods and time taken for the same is 6 seconds, if the detailed model would have run for 128 periods the time taken would have been more than 10 seconds.
* Comparing the cpu time even after running 128 periods of simulation FRM took only 37.79 min but the detailed model that run for 75 periods took 1:53:56, that is almost 2 hrs of cpu time.
* From the above result it is observed that for Sil, Hil and Mil type of simulations using FRM's will be a good option.