These characteristic reactor properties settings may be used in typical cases. For parameter studies of equivalence ratio or temperature, spread out the points roughly equally to cover the range of values.
Closed Homogeneous Reactor Problem Type
If the Batch reactor template is chosen, the following settings will be set automatically. If the template is not used and a Batch reactor model is set up from the beginning, the following inputs must be made:
Constant Volume and Solve the Energy Equation.
Adiabatic (heat loss = 0)
End time = 1 s. This is arbitrary, but is meant to set the simulation time long enough to cover one revolution of an engine crank and to cover the complete ignition event for the longest ignition-delay time of interest.
Operating Conditions:
Pressure:
Set the value to one that is within the range of pressures at TDC in the engine. For knock modeling, the pressure used should be set closer to the peak in-cylinder pressure of the normal cycle. However, the exact pressure is not required since the same kinetics will be captured when the pressure used is within a factor of 2 – 3 of that expected in the engine application. For this reason, no parameter study on pressure is needed—unless the range of application covers a wider range of pressures.
For diesel engines, a typical pressure of 40 atm is a good approximation. Similarly, for SI engines, a pressure of 20 bar is usually good. For gas turbines, operating pressure should be used. For applications involving a wide variation in pressures, two pressures covering the low and high limits in the application can be set in a parameter study.
Temperature:
Set a nominal value and then set up the parameter study for Initial Temperature. This is an important setting that varies with the use case.
Ignition in diesel engines: Typically, low-temperature kinetics are the most relevant in this case. The range of temperatures in the parameter study should cover temperatures between 700 K to 1200 K. It should be sufficient to select up to 4 temperature points in this range to represent the range of local temperatures prior to ignition in the engine.
For knock in SI engines: Combine ranges of settings similar to those recommended in both (a) and (c).
Flame propagation, or SI engines: Even though the temperature of the unburned mixture is low, only high-temperature kinetics are needed to represent the kinetics in the flame. A temperature range of 1200 K–2100 K is necessary to capture the flame kinetics. Select approximately 4 temperature points in this range.
Emissions: No additional range of temperatures is required to capture the emissions as they are inherently captured by the detailed reaction mechanism reduced for appropriate conditions in (a), (b), or (c).
Use the Equivalence Ratio option:
Set up a parameter study on equivalence ratio to cover the local conditions in the engine. Since the local equivalence ratio varies widely, it is important to cover the range of values for various use cases:
—For Diesel and SI engines: Use the range from 0.5 to 1.5. The parameter study should be adequate with 3 to 4 values in the parameter study. In some HCCI engines, the equivalence ratio may be highly fuel-lean; in such cases add the equivalence ratio of 0.3 to the parameter study.
—When PAH and soot emissions are among the targets, an equivalence ratio of 3 should be added to the parameter study.
Fuel composition: Specify that the fuel composition is the same as the model fuel composition to be used in the engine simulations.
Oxidizer and EGR: If you’re interested in only a small range of EGR, you can assume a median value of EGR rate and then use the fresh air charge plus EGR mixture (that is, IVC conditions minus the fuel) as the Oxidizer. If you want to consider EGR range, you can set up a parameter study with two extreme values (for example, 0 and maximum EGR level).
Example using One Oxidizer+EGR composition: For 40% EGR, we would calculate an “Oxidizer” composition in mass fractions, based on complete combustion of a typical H/C ratio for diesel:
CO2 = 0.0615, H2O = 0.0225, O2 = 0.1645, N2 = 0.7512.
To the above we can add a small amount of NO, for example, NO ~0.000005 – 0.000050, depending on emissions and EGR levels of the targeted engine.
Using a Parameter Study to vary EGR: To account for variations in the EGR in a Chemkin project, it is easiest to enter the EGR composition on the Added Species tab on the Reactant Species panel. Then, use pure air composition for the Oxidizer Mixture tab (use the auto-populate button). A parameter study is then set on the EGR species by doing a parameter study on the mass fractions of each of the Added Species. Typically just 2 extreme values (0 and maximum EGR) are needed.
Solver Parameters:
Accept defaults for everything.
Output:
With the Batch reactor template, the Temperature Inflection option is chosen (on the Output Control panel > Ignition Delay tab) for the Ignition criteria. If setting up the model from the beginning, you must specify this setting.
For typical applications, a parameter study with 4 temperature values and 4 equivalence-ratio values will contain a total of 16 runs in the Chemkin project. This number will grow when wider ranges of operation are represented by the parameter study. For example, it becomes 32 if you consider two pressures or two EGR levels. When multiple uses of the reduced mechanism are expected, use a wider range of conditions to cover all the specific use cases.