Diesel Sector Combustion Modeling with FORTÉ

Application Note: FORTÉ

Diesel Sector Combustion Modeling with
FORTÉ
FORTÉ-APP-Diesel Sector (v1.0) April 4, 2011

Overview
This applications note provides instructions for performing 3-D diesel-engine combustion simulations
with advanced spray models and accurate detailed chemistry. The simulation uses advanced
chemistry solution algorithms that include dynamic adaptive chemistry (DAC) and dynamic cell
clustering (DCC). The simulation employs a multi-component diesel-fuel surrogate mechanism with 437
species that was reduced for the conditions of interest from a comprehensive and well validated master
mechanism. The results show prediction of ignition behavior for low-temperature combustion
conditions, which provides good agreement with measured pressure and heat-release profiles. The
results also demonstrate some advantages of using a multi-component surrogate to capture
vaporization stratification within the engine cylinder.

Diesel Combustion Modeling Overview
Diesel engines are the workhorse of the transportation industry. Focus on improving diesel-engine
performance is therefore key to addressing regulatory objectives of reducing fuel consumption and
global warming gases. Low-temperature combustion regimes are characteristic of the advanced
combustion strategies being considered to improve performance. Such regimes promise greater
efficiency and fuel consumption with lower emissions. Low-temperature combustion regimes,
however, rely more critically on the kinetics of the fuel combustion than do more conventional operating
conditions. Accurate simulation therefore requires a detailed kinetics description for the fuel
combustion. The use of accurate in-cylinder combustion simulation offers the opportunity to reduce the
overall costs of developing clean diesel technologies, while decreasing the time to market for novel
designs.

FORTÉ Interface and Design Flow
This application note describes the use of a sector-mesh model to simulate diesel-engine in-cylinder
combustion, assuming axis symmetry based on the number of nozzle holes. The simulation uses a
multi-component surrogate-fuel model to capture both the complex kinetics of low-temperature
combustion and the vaporization and spray-break-up phenomena in the in-cylinder combustion
process. Details of the spray model set up are provided in the Multi-Component Spray Modeling with
FORTÉ Application Note.

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FORTÉ-APP-Diesel Sector (v1.0) April 4, 2011 www.reactiondesign.com 1
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Diesel Engine Modeling with FORTÉ

The FORTÉ CFD Package supports two tasks for modelers: Setup/Simulation and Visualize. Each task
has its own interface from the FORTÉ root directory. Figure 1 provides a map of the different areas
within the Setup/Simulation window, which provides a workflow tree that facilitates project setup and
simulation activities. A similar layout is used for the Visualizer window. In general, the project is set up
by working from top to bottom in the workflow tree, referencing geometry information displayed in the 3-
D view window, and filling in information in the panels displayed below the workflow tree for each
project-tree node.

Figure 1: Layout of the FORTÉ Setup window with the Define Simulation tab active

Toolbar
- Undo/Redo support for panel edits
- Forward/Back navigation for view changes
- Jump to common predefined views

Workflow tree
- navigates problem setup 3-D Display Area
- presents high-level view - pan/rotate/zoom with
of configuration intuitive & configurable
mouse actions

Visibility tree
- organizes display objects
Editor panels - allows fast color &
- organize user input into visibility changes
hierarchical groups
- unit sensitive
- XML-driven
(customizable)
+ / - buttons allow
showing/hiding detail

Log Window & Tooltip Area
- tooltips respond to Editor panels
- Log window relays status & messages

Test Case Description

For this application note, the model represents a single-cylinder, direct-injection (DI), 4-stroke diesel
engine based on a Cummins N-series production engine that has been extensively tested and
diagnosed at Sandia National Laboratories (Singh, et al, 2006). A schematic diagram of the engine is
shown in Figure 2, and the specifications of the engine are summarized in Table 1. The engine has a
bore of 139.7 mm and a stroke of 152.4 mm with a cylindrical cup piston bowl, yielding a displacement
of 2.34 liters for its one cylinder. The engine has a swirl ratio, which is the ratio of the flow rotation
speed to the engine rotation speed, of approximately 0.5 near top dead center (TDC).

The engine is equipped with a non-production, high-pressure, electronically-controlled, common-rail
fuel injector. Specifications for the fuel injector are included in Table 1. For the conditions modeled
here, an eight-hole, mini-sac injector cup (tip) was employed, having an included angle of 152° (14°
down-angle from the firedeck). The eight fuel orifices are equally spaced and have nominal diameter of
0.196 mm.

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Figure 2: Experimental Setup (Singh, et al, 2006)

Table 1: Engine and Injector Specifications and operating conditions (REF)

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Diesel Sector Model Setup in FORTÉ

To setup the problem in FORTÉ, we begin by reading in an existing mesh file and fuel mechanism and
then we will input the information required to setup and run the test case.

1. In the Editor panel, select the IC Engine option and check both Use Injector Spray Model and
Use Soot Model.

2. Import the sample case mesh Sandia.fmsh. You can see the geometry in the viewer window and
use the mouse to rotate, re-size and manipulate the view.

Figure 3: Diesel sector sample case

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3. Set up the Fuel Model:

• Import the fuel chemistry set Diesel_3compSurrogate_437species.cks, which is
located in the data directory of the FORTÉ install folder.

• Define the vaporization model for the fuel surrogate, as described in the FORTÉ User
Guide for this diesel-fuel mechanism, in the Fuel Spray Composition Editor panel. The
intended surrogate composition is 51% n-tetradecane, 35.5 n-decane, and 15.5 1-
methylnaphthalene. The corresponding species symbolic names for these species are:
nc14h30, nc10h22, and amn, respectively. .

• For each species added for the 3-component kinetics model, select the same
corresponding fuel name for the spray vaporization model, using the pull-down menus to
the left of the species. By default these are the same as the kinetics species selected.

4. Define the Simulation Settings:

• Set the simulation limits (starting and ending crank angles in this case) with the initial
crank angle at the intake-valve-closure (IVC) value of -165 degrees and the maximum
simulation crank angle at the exhaust-valve opening (EVO) value of 125 degrees.

• Accept the defaults for the fluid models (Turbulence Model, Wall Treatment, and Fluid
Properties).

5. Accept the defaults for the Solver Settings:

• The initial Time Step is set to 5.0E-7 seconds with 1.1 max crank angle degrees per time
step and a maximum simulation time step of 5.0E-6 seconds.

• The Chemistry Solver Options of Dynamic Adaptive Chemistry and Dynamic Cell
Clustering are selected.

• Chemistry Activation is set to start after fuel injection, since no chemistry will take place
before there is fuel to combust.

• Use the default Transport Term and Chemistry Solver Tolerances

6. Define the Boundary Conditions at the walls as 420K for the liner, 470K for the cylinder head and
500K for the piston.

7. Define the Nozzle properties and Fuel Injection events as described in the Multi-Component Spray
Modeling with FORTÉ Application Note.

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Figure 4: Initial Conditions

8. Define what results will be saved to the solution file and how frequently they will be saved in the
Output Control panel, as illustrated in Figure 5.
• Spatially-Resolved Results allow 3-D visualization of contour maps and spray patterns in
the Visualizer. Select which species data will be stored for each cell in the mesh as well
as the frequency in which data is written to the solution (*.ckres) file..

• Spatially-Averaged Results provide for X-Y plot visualizations, which are used to
generate the pressure vs. CA plots shown in the next section, for example. Select which
spatially averaged species data to save as well as the frequency in which it is written.
For better plot resolution, save the values at smaller crank-angle intervals.

• Restart Data can be saved at specified crank angles, intervals, or after a specified
number of timesteps, as well as at the end of the solution. This allows subsequent
simulations to be restarted from the saved location.

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Figure 5: Output Control: Spatially Resolved, Spatially Averaged and Restart panels

Results

The results for the 3-component, 437 species surrogate mechanism are compared against
experimental data of Singh, et al, 2006 in Figure 6. The pressure curve shows excellent agreement
with the experimental data. The heat release also shows good agreement with experimental data,
particularly for the timing of the initial heat release and the peak heat-release-rate value. The superior
combustion prediction of the 3-component surrogate is clearly seen when compared to the results for
pure n-tetradecane in Figure 7, where both simulations used the same detailed kinetics
mechanism.The pure n-tetradecane model ignites earlier due to the lack of stratification in fuel
vaporization and fuel reactivity. A more complete description of the techniques used to assemble and
reduce the surrogate mechanism is provided in (SAE 2010-01-0178). In this reference, you will also
find a comparison of another surrogate mechanism with a different component blend.

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Figure 6: Pressure and heat release rate for 437 species, 3-component surrogate mechanism

Figure 7: Pressure and heat release rate for pure n-tetradecane compared against experimental data

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Summary
In this application note, we presented how to use the FORTE CFD Simulation Package to analyze
diesel combustion in an engine using a multi-component accurate fuel surrogate with 437 species. The
guided set up of the engine conditions, fuel surrogate and initial conditions in the FORTE simulator
were also described. The results for ignition and heat release show good agreement with experimental
results.

References

Singh, S., Reitz, R., and Musculus, M., “Comparison of the Characteristic Time (CTC), Representative
Interactive Flamelet (RIF), and Direct Integration with Detailed Chemistry Combustion Models against
Optical Diagnostic Data for Multi-Mode Combustion in a Heavy-Duty DI Diesel Engine,” SAE Paper No.
2006-01-055, Detroit, Michigan, April 2006.

Long Liang, Chitralkumar V. Naik, Karthik V. Puduppakkam, Cheng Wang, Abhijit Modak, Ellen Meeks,
Hai-Wen Ge, Rolf D. Reitz, and Christopher J. Rutland, “Efficient Simulation of Diesel Engine
Combustion Using Realistic Chemical Kinetics in CFD,” SAE Paper No. 2010-01-0178, Detroit,
Michigan, April 2010.

About Reaction Design
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Technology goals by automating the analysis of chemical processes via simulation and modeling
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standard for modeling gas-phase and surface chemistry, providing engineers ultra-fast access to
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product brings accurate chemistry simulation to combustion systems using automated reactor network
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CHEMKIN® and Reaction Design® are registered trademarks of Reaction Design. CHEMKIN-PRO,
ENERGICO, and CHEMKIN-CFD are trademarks of Reaction Design.

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Diesel Sector Combustion Modeling with FORTÉ

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