Setting the Engine Models for a Combustion Simulation

Simulating the combustion process in Simcenter STAR-CCM+ In-cylinder requires the selection of combustion, ignition, and emission models.

To set the engine models for a combustion simulation:
  1. Right-click the Models node and select Edit.
  2. For the combustion of a liquid fuel, in the Model Selection dialog, select the models as described for a charge motion simulation in Setting the Engine Models.
  3. For both liquid and gaseous fuels, in the Optional Models group box, select Combustion.
  4. In the Combustion model group box, select one of the following models:
    • ECFM-3Z
    • ECFM-CLEH
    • Specified Burn Rate
    • Complex Chemistry
    The ECFM-CLEH model is faster than the ECFM-3Z model and, for Diesel engines, tends to be more accurate. For gasoline engines, both ECFM combustion models give similar results. The Specified Burn Rate model provides a cost-effective approach to obtain quick, first-cut solutions to multi-cycle CFD problems. The Complex Chemistry model can solve thousands of reactions among hundreds of species, but requires the import of complex chemistry definition files that contain detailed information about species, reactions, thermodynamics, and transport properties.

    Selecting the ECFM-CLEH model automatically selects the CO Emissions model, which accounts for the production of CO due to incomplete combustion.

  5. For a Complex Chemistry simulation, in the Chemistry Interactions group-box, select one of the following models:
    • Laminar Flame Concept—select this model for premixed, partially-premixed, and unsteady flames, or slow reactions.
    • Turbulent Flame Speed Closure—select this model for turbulent flames with premixed or partially-premixed flame fronts.
  6. Click Close.
  7. Depending on the selected combustion model, set up the model using one of the following procedures:
    Combustion ModelProcedure
    ECFM-3Z

    ECFM-CLEH
    1. Edit the Models > [combustion model] node, where [combustion model] is either ECFM-3Z or ECFM-CLEH.
    2. In the ECFM Model Time Setup group box, set the following properties:
      • ECFM Start Time
      • Combustion Reset Time—Simcenter STAR-CCM+ In-cylinder automatically sets this property to 5 degCA before intake valve opening (IVO). This value is suitable for most simulations.
      NoteFor multi-cycle runs using the ECFM-3Z model, the ECFM Start Time must be smaller than the Combustion Reset Time.

      See Models Reference—ECFM-3Z Dialog.

    3. For the ECFM-CLEH model, set the following properties:
      1. In the ECFM-CLEH Parameters group box, set the following properties:
        • Fuel
        • Premix Zone Transfer Coefficient (only for custom fuels)
        • Premix Transfer Burning Rate Limit (/s) (only for custom fuels)
      2. In the ECFM Equilibrium Table group box, set ECFM Equilibrium Table.
      3. In the CO Emission group box, set the following properties:
        • Pollutant Equilibrium Temperature Offset
        • Temperature of CO Model Cut Off
    4. Click Apply, then Close.

      See Models Reference—ECFM-CLEH Dialog.

    5. If you want to account for the formation of nitric oxide (NOx) during combustion:
      1. Edit the Models node and, in the Optional Models group box, select NOx Emission.
      2. Edit the Models > NOx Emission node and, in the NORA Parameters group box, set the following properties:
        • Cut Off Temperature
        • Input Fuel Enthalpy Correction
      3. In the NORA Table group box, select the NORA Table to import (*.tbl).
      4. Click Apply, then Close.

        See Models Reference—NOx Emission Dialog.
    Specified Burn Rate
    1. Edit the Models > Specified Burn Rate node.

      In the Graphics window, a Specified Burn Rate plot opens, which displays the default Wiebe function.

    2. In the Specified Burn Rate Settings group box, Simcenter STAR-CCM+ In-cylinder automatically sets Combustion Reset Time for multi-cylce runs to 5 degCA before exhaust valve opening (EVO). This value is suitable for most simulations.
    3. To modify the default Wiebe function, set the following properties:
      • Exponent
      • Timing at 50% Fuel Burned
      • Duration of 10-90% Fuel Burned

      The Specified Burn Rate plot updates to display the specified curve.

    4. Click Apply, then Close.

      See Model Reference—Specified Burn Rate Dialog.
    Complex Chemistry
    1. Right-click the Models > Complex Chemistry nodel and select Edit.
    2. In the Chemkin Files group box, click Import Chemkin Files....
    3. In the Import Chemkin Files dialog, do the following:
      1. Select the Chemical Mechanism File and the Thermodynamic Properties File to import.

        For more information, see Reaction Mechanism Formats.

      2. If you are simulating a laminar flame and the molecular transport properties are important, make sure that the Import Transport Properties File option is activated, then select the Transport Properties File to import.
      3. If you are simulating a turbulent flame and the turbulent transport dominates over the molecular transport, deactivate the Import Transport Properties File option.
      4. Click OK.
    4. If you use the Turbulent Flame Speed Closure model and want to relax species to their equilibrium compositions, in the Approximation Options group box, activate Relax to Chemical Equilibrium, then set the Timescale Constant.
    5. Depending on whether or not you selected the Relax to Chemical Equilibrium option, do one of the following:
      Relax to Chemical Equilibrium Procedure
      Selected In the Chemistry Acceleration group box, if required, activate Clustering.
      Not selected
      1. In the Chemistry Acceleration group box, if required, activate the necessary acceleration options:
        • Clustering
        • Dynamic Mechanism Reduction
      2. In the Chemistry Solver Tolerances group box, set the following properties
        • Absolute Tolerance
        • Relative Tolerance
    6. Click Apply, then Close.

      See Model Reference—Complex Chemistry Dialog.

    7. For the Turbulent Flame Speed Closure model, define the model properties:
      1. Edit the Models > Turbulent Flame Speed Closure node and specify the model properties, see Turbulent Flame Speed Closure Dialog.
      2. Click Apply, then Close.
  8. For an ECFM combustion model, to start combustion and to model the ignition process, select an ignition model:
    1. Edit the Models node and select the following models:
      Group Box Model
      Ignition Model Select one of the following models:
      • Spark Ignition—select this model for spark ignition internal combustion engines.
      • Auto Ignition—select this model for internal combustion engines where the combustible mixture ignites spontaneously, such as for Diesel engines.
      Spark Ignition Model

      (if the Spark Ignition engine model is selected)

      		 Select one of the following models:
      • FI Spark-Ignition
      • ISSIM Spark-Ignition

      The ISSIM Spark-Ignition model provides a more accurate description of the physics associated with ignition and initial flame kernel development than the more basic FI ignition model.
      For a Complex Chemistry simulation, you can select the Spark Ignition model as an optional model. The ISSIM Spark-Ignition model is then selected automatically.
    2. For the ISSIM Spark Ignition model, set the model parameters as follows:
      1. Edit the Models > ISSIM Spark-Ignition node.
      2. In the Physics group box, set the following propeties:
        • Initial Burnt Mass
        • LFS Sphere Coefficient
        • Initial Burnt Mass Coefficient
        • Spark Energy Dilution Coefficient
      3. For the Laminar Flame Speed Correction, set the Maximum Unburnt Temperature.
      4. For the evaluation of early ignition flame propagation, you can solve the flame kernel radius from a 0D equation. This option is recommended for models without a Flame Surface Density (FSD) equation, such as Complex Chemistry.

        To solve for the 0D flame kernel equation, activate Solve 0D Equation For Flame Kernel Radius and set the following properties:

        • Maximum Wrinkling Coefficient
        • Wrinkling Production Coefficient
        • Transition Radius
        • Maximum Transition Radius
      5. For a Complex Chemistry simulation that uses the Laminar Flame Concept, in the Laminar Flame Speed group box, select one of the following options for controlling the unstrained laminar flame speed:
        • Gulder
        • Metghalchi
      6. Specify the Fuel, which determines the values for the fuel-dependent model coefficients.
      7. Click Apply, then Close.

        See Models Reference—ISSIM Spark-Ignition Dialog.

  9. For the Spark Ignition model, if you want to account for the spontaneous ignition of the combustible mixture:
    1. Edit the Models node and, in the Optional Models group box, select Knock.
    2. Edit the Models > Knock node and in the TKI Parameters group box, set the following properties:
      • Delay Factor
      • Burn Rate Factor
      • Dual Zone TKI Model Option
    3. In the TKI Table group box, select the TKI Table to import (*.tbl).
    4. Click Apply, then Close.
    See Models Reference—Knock Dialog.
  10. If you want to account for an uneven distribution of fuel mass in a cell during the evaporation of a liquid fuel (only for an ECFM combustion model together with the Auto Ignition or the Knock model):
    1. Edit the Models node and, in the Optional Models group box, select Fuel Saturation Distribution.
    2. Click Apply, then Close.
  11. If you want to account for the formation of soot during combustion (only for ECFM and Complex Chemistry combustion models):
    1. Edit the Models node and, in the Optional Models group box, select Soot Emissions.
      The Soot Sectional model is selected automatically.
    2. Click Close.
    3. Edit the Models > Soot Sectional node and, in the Soot Properties group box, set the following properties:
      • Nucleation
      • Steric Factor Alpha
      • Scales for Surface-growth, Nucleation, Oxidation, and Coagulation
      • Number of Sections
      • Maximum Soot Diameter
    4. For an ECFM combustion mode, in the Soot Table group box, select the soot table to import.
      You can create soot tables using DARS.
    5. Click Apply, then Close.
      See Models Reference—Soot Sectional Dialog.
  12. Continue with Step 2 in Setting the Engine Models.