Full Hydro Simulation: SI8-PFI Engine
OBJECTIVE:
The objective of this study is to set up and run a full hydrodynamic simulation of a SI8-PFI engine model and calculate the combustion efficiency, power and torque produced by the engine based on the simluation results
The following parameters shall be considered for setting up the case
Engine Geometric Parameters
- Bore =0.086 m
- Stroke = 0.09 m
- Connecting rod length = 0.18 m
- RPM = 3000
Run parameters
- Simulation mode: Transient
Simulation time parameters
- Start time : -520 deg
- end time : 120 deg
Boundary conditions
- Piston temperature - 450K
- Liner temperature - 450K
- Head temperature - 450K
- Spark Plug temperature - 550K
- Spark Plug electrode temperature - 600K
- Exhaust ports temperature - 500K
- Exhaut outflow - 1 bar
- Exhaust outlflow temperature - 800k
- Exhaust species concentration - Stoichiometric Condition
Calculation for the stoichiometric condition
The general reaction Equation would be
C8H18+(O2+3.76N2)->CO2+H2O+N2
Now balancing the reacting species on both side we get-
C8H18+12.5(O2+3.76N2)->8CO2+9H20+12.5*(3.76N2)
Now, the molecular weight of CO2=44.01g/mol, H2O=18.01528, N2=28.0134
As per the equation, the mass of CO2=(molecular weight*moles) i.e., 44.01*8=352.08
Similarly for H2O=162.1375 & N2=1316.63
Now, the Total mass of the products= 1830.847
Mass Fraction of CO2=352.08/1830.847=0.192304
Mass Fraction of H2O=162.1375/1830.847=0.088559
Mass Fraction of N2=1316.63/1830.847=0.719137
- Exhaust valve top temperature - 525K
- Exhaust valve angle temperature - 525K
- Exhaust valve bottom temeprature - 525K
- Intake port - 1 temperature 425K
- Intake port - 2 temperature 425K
- Inflow pressure - 1 bar total pressure
- Inflow temperature - 363K
- Inflow species - Air
- Intake valve top temperature - 480K
- Intake valve angle temperature - 480K
- Intake valve bottom temeprature - 480K
Initial conditions
- Intake port - 1 (closer to combustion chamber)
- ic8h18 - 0.025508
- o2 - 0.20157
- n2 - 0.77292
- Temperature - 390k
- pressure - 1 bar
- Intake port - 2 (away from combustion chamber)
- Air
- Temperature - 370K
- Pressure - 1 bar
- Cylinder
- Pressure - 1.85731 bar
- Temperature - 1360K
- Stoichiometric composition
Injection Parameters
- Fuel flow rate = 3.0e-4 Kg/second
- Injection start time = -480.0
- Injection duration = 191.2
- Fuel temperature = 330K
Nozzle positions
Nozzle diameter = 250 micro-meter
Circular injection radius = Nozzle radius
Spray cone angle = 10
- Nozzle 0
- center 0.0823357 0.00100001 0.07019
- Align Vector -0.732501 0.210489 -0.647408
- Nozzle 1
- center 0.0823357 -0.00099999 0.07019
- Align Vector -0.732501 -0.210489 -0.647408
- Nozzle 2
- center 0.0823357 -0.0004 0.07019
- Align Vector -0.5 -0.2 -0.647408
- Nozzle 3
- center 0.0823357 0.0003 0.07019
- Align Vector -0.5 0.2 -0.647408
Spark Ignition Parameters
- Start of spark = -15 deg
- Spark duration = 10 deg
- Spark location = -0.003 0 0.0091
- Spark radius = 0.0005m
The simulation setup process includes three major steps:
1. Preprocessing
1.1. Geometry Import
1.2. Diagnosis
1.3. Boundary Flagging and Surface Preparation
1.4. Case Setup
2. Solving
3. Post Processing
3.1. Plots
3.2. Animation
Note: The boundary flagging and surface preparation are similar to the no hydro setup, the link to which is given below
PROCEDURE:
1. Preprocessing
1.1. Geometry Import:
The SI8-PFI engine model was directly imported into converge studio for setting up the full hydro case.
Fig: SI8-PFI Engine Geometry
1.2. Diagnosis: The diagnosis tool helps in identifying the different problems in the geometry. For performing the diagnostics on the model goto Diagnostics Dock>Find>
The Diagnosis tool looks for the following geometric issues
To reduce the time for setting up the case, the input files generated by the No hydro case setup was loaded into this case setup for ease. Since the boundary flagging and surface preparation process were similar loading the similar input files eleminates the repetitive task
Fig: Regions Flagged
Fig: Surface with Edges
Note:
1. The boundaries shown above are coloured by region
2. The purpose of creating two inlet ports was that near to the cylinder region the temperature of the inlet port is higher due to conduction as compared to the port region which is located farther from the cylinder
1.3. Case Setup:
Since all the input files from no hydro case was imported, case settings of no hydro simulation got uploaded onto the converge studio case setup wizard for full hydro. For setting up the full hydro the various changes were made in the case setup and below are the final comprehensive setup values/options opted
| Settings | Values to be Entered/Selected | ||||||||||||||||||||||||||||||||||
| Application | IC-Engine (Crank angle-based) | ||||||||||||||||||||||||||||||||||
| Material |
Tick on Reaction Mechanism, Parcel Simuation and Species |
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| Simulation Parameters |
Run Paramters>Simulation Mode>Select Full Hydrodynamic |
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| Boundary Conditions |
Piston:
Velocity Boundary Conditions>Wall Motion type>stationary. select 'Law of Wall' in both velocity and temperature boundary condition, temp value>800k
Temperature Boundary Conditions: 1. The direction values entered for the intake and exhaust valve boundaries (top, angle and bottom) can be obtained from the geometry. For this goto measure>direction>arc normal>select three points on the value shaft top's cross sectional area and click on apply. A message will be displayed on the message log window with the normal values, copy this value and paste it to the valve normal. The valve normal for inlet and outlet valves shall be different as both the set of valves will be moving along different axes 2. Disconnect Triangles: these traingles are created to control the fluid flwo different two different region. The region around the inlet/exhaust valve angle and the cylinder in one such are where these triangles are created, as the flow between these two regions is established during the intake and exhaust cycle only, for the other cycles: compression and power stroke cycle, there is no flow between these region. So the minimum lift be set in such as way that the following conditions are fulfilled a. It should not be too big so that very large size disconnect traingles are formed b. While opening the valves should not collide with the cylinder head/piston Assign the regions to the boundaries as per the following:
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| Initial Conditions & Events |
Passive: Add HIROY_SOOT ant NOX, assign 1 to the value of both Passive: Add Intake and put the value 1 Passive: Add Exhaust, HIROY_SOOT, NOX and assign value of 1 to individual ones Events |
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| Physical Models |
Activate Spray Modelling, Combustion Modelling & Source/Sink Modelling from the Menu Turbulence Modeling-RNG-K Eplison is selected Spray Modelling: Under the general Tab the following settings were opted Collision/Breakup/Drag tab the following settings were opted Wall Interation Tab Settings Injectors Tab Settings Select Injector 0 and click on edit for further configuration. Below is the image for the same Click on plot to check the injection rate graph and it should look as given below Close the above window and goto Model tab for the below given settings Goto Time/Temp/Mass/Size and provide the setting given below Goto Nozzle and use the following configurations Select Nozzle 0 and click on edit to enter the nozzle configuration Similarly for the other 3 nozzles enter the values given below Nozzle diameter = 250 micro-meter Circular injection radius = 125 micro-meter Spray cone angle = 10
Combustion Modelling Goto combustion modelling and provide the following settings Goto Emissions and provide the following settings Goto General tab and provide the following settings Source/Sink Modelling The source/sink model is used to provide the spark parameters. Here we need to add 2 sources as a real spark takes place in 3 stages: Breakdown, arc and blow. At around 0.5 crank degrees the spark initiation takes place which emits around 40 mJ of heat post which arc formation takes place followed by blow. After the initiation phase less energy is required so in the arc and blow phase around 20 mJ of heat is emitted for spark continuation in these 2 phases Source 2 Motion- same as source 1 |
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| Grid Controls |
Activate Fixed Embeddding and Adaptive Mesh Refinement from the menu Base Grid: Adaptive Mesh Refinement: Add AMR Group1, select inlet_port1, inlet_port2 and cylinder to active regions and goto the temperature tab Enter the following values In the velocity tab enter the following values Note: Adaptive mesh refinement is added to capture the physics at these regions with greater accuracy Fixed Embedding- Intake_Valve_Angle: BigCylinderEmbed Entity Type: Cylinder In the Center of Cylinders add the following: Center 1 & 2- (0.0,0.0,0.08): (0.0,0.0,-0.15) SmallCylinderEmbed Entity Type: Cylinder In the Center of Cylinders add the following: Center 1 & 2= (0.0,0.0,0.018): (0.0,0.0,-0.15) Spherical: Period: 720 degree Start Time:-16 degree End Time: 7 degree Center of Sphere (-0.003, 0.0, 0.0091) Radius=0.001m Large Sphere: Period: 720 degree Start Time:-16 degree End Time: 7 degree Center of Sphere (-0.003, 0.0, 0.0091) Radius=0.003m Note: 2 AMRs are added at the spark location for better capture of the physics during the spark duration Injector Embedding: Mode: Cyclic Period: 720 degree Start Time:-482 degree End Time: -288 degree InjectorID: Inector 0 Radius 1=0.002m Radius 2=0.004m Length=0.02 |
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| Output/Post Processing |
Post Variable Selection> Typical Parameters/Geometry>No Change Species/Passives: Species>Add Mass Fraction for N2, CO, CO2, NO, NO2, H2O, IC8H18 Passives>NOX, HIROY_SOOT Soot>Parcel: tick on mass fraction |
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| Output Files |
Time Interval for writing 3D output data files: 2 deg |
Fig: Geometry with Embedded and Nozzles
Fig: Timing Map
The timing map highlights all of the events of mesh refinement and fixed embedding timing for the 720 degree cycle
After this all the input files was imported to a seperate folder. Converge creates .txt files which contains all the values entered while setting up the case for respective settings done above. These input files will be processed by converge using Message Passing Interface (MPI) standard
2. Solving:
For running the simulation, Cygwin tool was used instead of converge studio interface. Cygwin is a POSIX-compatible API which is based on command prompt is used to run the simulation
Steps:
a. Navigated to the folder location containing the input files
b. Entered the command> mpiexec.exe -n 2 converge.exe </dev/null> logfile & : This allows converge to run two processors (selection on number of processor cores present as per the system configuration, can use multiple cores if available) for the parallel processing and load balancing
c. Entered the command> taif -f logfile
d. After the simulation is over. Went back to converge studio>Post-Processing 3D
e. Enter Case Name>"Test", Change the File Type to Paraview VTK in-line binary format
d. Enter the address for the output files>Select all files>Select all Cell Variables>click on Convert
e. After the conversion of the post files to the binary output, open the Test.vtm (group file) file in Paraview
3. POST-PROCESSING:
Plots
Fig: Volume (M^3) vs Crank Angle (Degree)
Fig: Cell Count Vs Crank Degree
Fig: Emission Species- Soot, NOx, CO, CO2, HC(Kg) vs Crank Angle (Degree)
Fig: Integrated Heat Rate (Joule) vs Crank Angle (Degree)
Fig: Maximum Pressure (MPa) Vs Crank Angle (Degree)
Fig: Mean Temperature (K) vs Crank Angle (Degree)
ANIMATION:
No Hydro Simulation: SI8-PFI Engine Isometric View
No Hydro Simulation: SI8-PFI Engine Cutplane View
Full Hydro Animations:
Exhaust: SI8-PFI Engine
Temperature Contour Animation: SI8-PFI Engine
Velocity & Flame Front Temperature Distribution Contour Animation: SI8-PFI Engine
yC8H18 & Pressure Contour Animation: SI8-PFI Engine
KEY CONCLUSIONS:
1. Compression Ratio: It is the ratio of total swept volume of the cylinder covered by the piston travelling fromTDC to BDC to the clearance volume or the total volume compressed with piston at the TDC
Compression Ratio: Maximum Volume/Minimum Volume= 5.74x10^4/5.701x10^5 (Both the values are acquired from the volume vs crank angle graph)=10.07:1
2. Combustion Efficiency: It is defined as the ratio of energy released by the burning the fuel to the theoritical energy content of the fuel mass during one complete cycle
Combustion Efficiency= Heat Released/Heat Content of the Fuel
Heat content= Fuel Mass inducted*CV of fuel=(3.0x10^-5 )x(44x10^6)=1320 J
Heat Released=1241 J (from the Integrated HR graph)
Combustion Efficiency=(1241/1320)x100= 94.015%
Significance of Wall Heat Transfer Model: Wall heat transfer model is used to the
3. Power and Torque:
In converge a engine performance calculator tool was used for this
Work=468.646 Nm
Total Degree=120.11- (-120.09)=240.2 Degree
Engine speed = 3000 RPM or 50 RPS
For 1 rotation (360 degree), time= 1/50= 0.02 sec
For 240.2 degree, time = 240.2x0.02/360=0.013344 sec
Power =Work/Time=468.464/0.013344= 35,106 W or 35.1 KW
Power Output=2`pi`NT/60 or T= Px60/(2`pi`N)=35106x60/(2x3.14x3000)=111.8 Nm
Theory:
Significance of Wall Heat Transfer Model: In any CFD simulation, the Navier stokes equation is solved for getting the results. The complexity of the equation increases significantly as the simulation case gets more complex and therefore more time is required to solve the equations. To reduce the complexity of the equation various models are used which simplifies the equation thereby reducing the time required for the same, without compromising the result effectiveness or rather capture near to experimental results. Wall heat transfer model is one such model which is used to capture the physics near the wall boundaries. In case this wall heat transfer model is not used the solver will simulatenous solve the heat transfer equation as well as the NV equation which can consume significant amount of time. Apart from this the selection of these models also depend on the type of simulation which is being run, in cases where conjugate heat transfer is a mandatory consideration, the concept of supercycling can be used alongside wall heat transfer model, else only the wall heat transfer model can do the predict the desired accuracy in the results. As in the SI8-PFI engine case, the focus or the domain is limited to within the cylinder and the ports, here heat transfer through the cylinder chest (conduction heat transfer) is not of prominent concern so wall heat transfer model is used to capture the heat transfer physics within this domain. If the cylinder/engine chest cooling has to be considered and were of the key interest then the supercycling concept could have been used alongwith the wall transfer model
Significance of CA10, CA50 and CA90:
CA10, CA50, CA90 are combustion phases which refers to the corresponding crank angle traversed for 10%, 50% and 90% of the total heat released during the combustion respectively. The pressure, temperature, premix ratio and other parameters are collected for these angles and compared with the experimental results, which can be further used to predict the combustion lag for engine output optimization.
For the current setup the following are the values
CA10: 6.8371 Degrees
CA50:18.4623 Degrees
CA90: 31.701 Degrees