This note describes how the FORTÉ Simulation Package can be used to include detailed chemistry in internal combustion engine simulations. The enhanced chemistry solution techniques in FORTÉ allow detailed chemistry to be efficiently included in the FORTÉ computational fluid dynamics (CFD) calculation. These enhancements allow designers to accurately predict ignition, emissions, combustion duration, and engine performance without sacrificing geometric fidelity and without compromising accuracy for solution efficiency.

1. Application Note: FORTÉ
HCCI Engine Performance Evaluation Using
FORTÉ Simulation with Detailed Chemistry
FORTÉ-APP-HCCI Engine (v1.0) October 13, 2011
Overview
This application note describes how the FORTÉ Simulation Package can be used to include detailed
chemistry in internal combustion engine simulations. The enhanced chemistry solution techniques in
FORTÉ allow detailed chemistry to be efficiently included in the FORTÉ computational fluid dynamics
(CFD) calculation. These enhancements allow designers to accurately predict ignition, emissions,
combustion duration, and engine performance without sacrificing geometric fidelity and without
compromising accuracy for solution efficiency.
Overview of HCCI Engines
Homogeneous charge compression ignition (HCCI) engines are a promising engine technology due to
the reduced emission output and increased efficiency associated with these types of engines. The
basic concepts behind HCCI engine technology have been known for quite some time, but there have
been many technical challenges in the practical implementation of these concepts. With increased
emissions standards and the need to reduce oil consumption, a renewed focus has been placed on this
promising technology. To overcome the barriers, however, in depth understanding of the HCCI regime
of combustion and the sensitivity of the combustion performance to operating conditions is of increasing
interest.
In spark-ignition engines, an external source is used for ignition, whereas in an HCCI engine, there is
no external source, which means that the timing of the combustion is not explicitly controlled by an
external event. Instead, in the HCCI engine the fuel-air mixture will auto-ignite when suitable conditions
in the combustion chamber have been achieved. The control of ignition in HCCI engines with varying
load conditions has been the most challenging aspect of this technology, and thus is the primary reason
it has not been widely used. To control ignition in an HCCI engine, one must control the compression
ratio, fuel-air ratio, inducted gas-temperature, as well as other parameters. With the improvements in
micro-processor and control technology, this previous control barrier has been reduced, furthering the
interest in HCCI engines.
This application note focuses on using FORTÉ to accurately predict the ignition behavior
simultaneously to predicting the engine-out emissions of an HCCI engine using CFD in combination
with detailed chemistry and a realistic fuel surrogate. In traditional CFD packages, the chemistry
mechanisms used are severely reduced and/or a look-up table approach is used, which leads to
significant inaccuracies in ignition and emission predictions and eliminates the possibility of studying
©Reaction Design. All rights reserved. All Reaction Design trademarks, patents, and disclaimers are listed at www.reactiondesign.com.
All other trademarks and registered trademarks are the property of their respective owners. All specifications are subject to change without notice.
FORTÉ-APP-HCCI Engine (v1.0) October 13, 2011 www.reactiondesign.com 1
1-858-550-1920

2. HCCI Engine Performance Evaluation using
FORTÉ Simulation with Detailed Chemistry
fuel effects on either of these phenomena. With the advanced chemistry solvers and techniques in
FORTÉ, detailed finite-rate chemistry can now be included in CFD computations efficiently for accurate
prediction of emissions and important combustion phenomena, such as ignition timing, combustion
duration and fuel effects.
Model Overview and FORTÉ Setup
In this application note, we describe CFD simulations of an Oak Ridge National Laboratory (ORNL)
gasoline HCCI engine [1], in which a large number of diagnostic measurements have been made. The
engine experiments used an E30 gasoline surrogate blend and measured several trace gas species at
the exhaust [2,3]. The simulations focus on the inclusion of detailed finite-rate chemistry to predict
ignition and emissions.
The surrogate fuel used in both the model and experiments consists of 33% ethanol, 8.7% n-heptane,
and 58.3% iso-octane by weight. The detailed chemical kinetic mechanism used in the simulations
consists of 428 species and 2378 reactions. This mechanism was obtained by using a targeted
mechanism reduction of a well validated master kinetics mechanism for multiple gasoline surrogate-fuel
components, which consists of 3553 species and 14904 reactions [4]. The number of cells varied from
53,800 at intake valve close (IVC) to 10,600 at top dead center (TDC), which results in a total wall clock
time of approximately 17 hours using eight processors, which is reasonable for practical use in engine
design. Additional processors can reduce the simulation time, with an approximately linear scaling
between simulation time and the number of processors.
Details of the engine are shown in Table 1.
Table 1. ORNL Engine Specifications
Engine Specifications
Fuel Injection Port atomization
Geometric C.R. 14.5:1
Displacement (cm3) 517
Bore x Stroke (cm) 9.7 x 7
Connecting Rod Length (cm) 11
Bowl width, depth (cm) 8.7, 0.74
FORTÉ-APP-HCCI Engine (v1.0) October 13, 2011 www.reactiondesign.com 2
1-858-550-1920

3. HCCI Engine Performance Evaluation using
FORTÉ Simulation with Detailed Chemistry
IVC, EVO (CAD ATDC) -142, 139
Since there are no valves or spray-injection events with this case, the model can be simplified to a
sector (slice) of the engine. In this case, a one degree sector mesh is used for the CFD case. By
modeling a sector of the geometry, the computational overhead is greatly reduced and improves turn-
around time for running the case. The sector mesh consists of 53,800 computational cells with a mesh
length of 0.7mm or lower in the core zone, and a well resolved mesh near the boundaries and crevice.
A snapshot of the computational model with the mesh displayed is shown in Figure 1. If the case is not
symmetric or the intake and outtake valves are included, then the sector mesh simplification would not
be valid and the full three dimensional geometry would need to be modeled.
Figure 1. CFD Model with Mesh Displayed
One of the many defining features of FORTÉ are its advanced chemistry solving techniques. These
techniques allow the inclusion of detailed chemistry without sacrificing solution performance. The
techniques employed include dynamic adaptive chemistry (DAC) and dynamic cell clustering (DCC)
methods, as well as well as other proprietary advances. The DAC and DCC methods are discussed in
more detail below.
FORTÉ-APP-HCCI Engine (v1.0) October 13, 2011 www.reactiondesign.com 3
1-858-550-1920

4. HCCI Engine Performance Evaluation using
FORTÉ Simulation with Detailed Chemistry
Detailed reaction mechanisms consists of hundreds (or thousands) of species and thousands of
reactions, which allows them to make accurate predictions over a wide range of operating conditions,
such as pressure, equivalence ratio and temperature. However, only a small subset of these species
and reactions are relevant at any given point in time and space during the solution process, thus
creating an unnecessary computational overhead. Using dynamic adaptive chemistry in FORTÉ, the
detailed mechanism is reduced on-the-fly during the CFD computation. By analyzing the local
conditions of the system for a given time step and a given computational cell, the detailed mechanism
is reduced to obtain an appropriate set of species and reactions. Using DAC, the number of species
and reactions in play at any given time is reduced drastically.
Another method available to reduce the chemistry computation time in FORTÉ is the dynamic cell
clustering method. Typically, the chemistry equations are solved on a cell-by-cell basis during the CFD
computation. Given that the chemistry portion of the equation set is independent of the mass and
volume of a cell, it is possible to cluster together any cells that have kinetically similar properties, such
as temperature, pressure, and composition [5]. By clustering cells with similar properties, the typical
CFD cell-by-cell solution process is reduced to the number of unique clusters in the system. This
technique further reduces the computational overhead when using detailed chemistry. Figure 2 shows
the inputs for DAC and DCC in FORTÉ.
FORTÉ-APP-HCCI Engine (v1.0) October 13, 2011 www.reactiondesign.com 4
1-858-550-1920

5. HCCI Engine Performance Evaluation using
FORTÉ Simulation with Detailed Chemistry
Figure 2. Dynamic Adaptive Chemistry and Dynamic Cell Clustering Inputs in FORTÉ
In FORTÉ, the DAC and DCC methods can be used simultaneously for improved chemistry calculation
times without sacrificing accuracy. The speedup and accuracy are shown in the results section of this
application note.
FORTÉ HCCI Simulation Results
While prediction of emissions is essential, the accurate prediction of ignition and combustion duration is
also of particular importance. Figure 3 shows a comparison of predicted and measured pressure
profiles for three different IVC temperatures. The FORTÉ simulation results show the correct trends for
FORTÉ-APP-HCCI Engine (v1.0) October 13, 2011 www.reactiondesign.com 5
1-858-550-1920

6. HCCI Engine Performance Evaluation using
FORTÉ Simulation with Detailed Chemistry
both ignition and combustion duration when compared with the experimental results. In the 494K IVC
case, ignition occurs before TDC, while in the 472K case it occurs after TDC [1].
Figure 3. Comparison of predicted pressure profiles with experimental data. Dotted lines are engine data and solid lines
are FORTÉ predictions
In Figure 4, results are shown for combustion duration, mean fuel burn at 50% heat release (MFB50),
peak pressure, and mean fuel burn at 10% heat release (MBF10) for all three IVC temperatures. The
model predictions and data showed increasing combustion duration with retarded combustion phasing
(MBF50). The predicted trends for the model results agree well with the data, although there is some
difference at the lowest IVC temperature (472K).
Figure 4. Combustion duration, MFB50, MFB10, and peak pressure comparisons with experimental data
Detailed engine exhaust measurements were provided by ORNL for comparison to the simulation
results [2,3]. Of particular importance are the concentrations of NOx, CO, and unburned hydrocarbons
FORTÉ-APP-HCCI Engine (v1.0) October 13, 2011 www.reactiondesign.com 6
1-858-550-1920

7. HCCI Engine Performance Evaluation using
FORTÉ Simulation with Detailed Chemistry
(UHC). Figure 5 shows a comparison of NOx, CO, and UHC with varying combustion phasing that was
achieved by varying IVC temperatures. The results show higher UHC and CO concentrations at lower
IVC temperatures. The NOx concentrations are relatively low in this engine due to the fuel lean
conditions. In all cases, the simulation results compare well with the experimental data. Not only are
the trends captured, the detailed chemistry in the simulation accurately predicts the absolute values,
within the experimental uncertainty for such measurements.
Figure 5. Comparison of FORTÉ Simulation Results with Experimental Data for Unburned Hydrocarbons, NOx, and CO with
varying IVC Temperature
Data Model
UHC NOx
5000 40
4000
3000
ppm
ppm
20
2000
1000
0 0
460 470 480 490 500 460 470 480 490 500
IVC Temperature (K)
IVC Temperature (K)
CO
2500
2000
1500
ppm
1000
500
0
460 470 480 490 500
IVC Temperature (K)
While CO, NOx, and UHC are of particular interest, the inclusion of detailed chemistry in the FORTÉ
simulation allows for the prediction of other trace species. Figure 6 shows the prediction of
formaldehyde and acetaldehyde as a function of IVC temperature with the trends in agreement with the
data.
Figure 6. Comparison of model predictions for formaldehyde and acetaldehyde as a function of IVC temperature
FORTÉ-APP-HCCI Engine (v1.0) October 13, 2011 www.reactiondesign.com 7
1-858-550-1920

8. HCCI Engine Performance Evaluation using
FORTÉ Simulation with Detailed Chemistry
The model predictions were obtained using a 428 species, 2378 reaction mechanism in conjunction
with the dynamic adaptive chemistry (DAC) and dynamic cell clustering (DCC) options available in
FORTÉ. The DAC option performs an on-the-fly mechanism reduction during the simulation which
results in fewer active species during a time-step, as shown in Figure 7.
Figure 7. Number of Active Species During the Simulation using Dynamic Adaptive Chemistry (DAC)
500
Active number of
400
species Master Maximum
300 mechanism
200
100
Average Minimum
0
-50 0 50
Crank angle
The average number of species during the calculation is approximately half of the 428 species
mechanism, which significantly increases the speed of the computation. Using DCC, the computational
time is further reduced by lumping cells of similar thermo-chemical states before solving the kinetic
equations. Figure 8 shows the number of cells in the CFD model and the number of clusters used
during the simulation.
Figure 8. Number of Cells in the Model and Number of Clusters Used for Chemistry Calculations
Number of cells/clusters
10000
1000 Number of
cells
100
Number of clusters
10
-50 0 50
Crank angle
The total number of clusters used to solve for chemistry is an order of magnitude smaller than the total
number of cells in the computational domain. This results in a significant increase in computational
savings.
FORTÉ-APP-HCCI Engine (v1.0) October 13, 2011 www.reactiondesign.com 8
1-858-550-1920

9. HCCI Engine Performance Evaluation using
FORTÉ Simulation with Detailed Chemistry
Summary
The inclusion of detailed chemical kinetics in CFD simulations is essential for accurately predicting
emissions concentrations and combustion performance. In the past, the solution time for including
detailed chemistry was prohibitive for use in the commercial design market. With the advanced solution
techniques in FORTÉ, e.g., dynamic adaptive chemistry and dynamic cell clustering, this limitation has
been greatly reduced. These improvements increase the computational efficiency and accuracy for
engine design, which leads to improved understanding of combustion phenomena and shortened
design cycle times.
References
[1] Puduppakkam, K., et al., Predicting Emissions Using CFD Simulations of an E30 Gasoline
Surrogate in an HCCI Engine with Detailed Chemical Kinetics. Submitted to the SAE 2010 Congress,
10PFL-0708, 2010.
[2] Bunting, B., et al., Detailed HCCI Exhaust Speciation – ORNL Reference Fuel Blends, in Directions
in Engine-Efficiency and Emissions Research (DEER) Conference. 2009.
[3] Bunting, B.G., et al., A Comparison of HCCI Engine Performance Data and Kinetic Modeling
Results over a Wide Range of Gasoline Range Surrogate Fuel Blends, in Directions in Engine-Efficiency
and Emissions Research (DEER) Conference. 2009.
[4] Naik, C.V., et al., Applying Detailed Kinetics to Realistic Engine Simulation: the Surrogate Blend
Optimizer and Mechanism Reduction Strategies. Submitted to the SAE 2010 Congress, 10PFL-0131,
2010.
[5] Liang, L., et al., Efficient Simulation of Diesel Engine Combustion Using Realistic Chemical
Kinetics in CFD. Submitted to the SAE 2010 Congress, 10PFL-0056, 2010.
About Reaction Design
Reaction Design, a San Diego, California-based software supplier, enables transportation
manufacturers and energy companies to rapidly achieve their Clean Technology goals by automating
the analysis of chemical processes via computer simulation and modeling solutions. Reaction Design is
the exclusive developer and distributor of CHEMKIN, the de facto standard for modeling gas-phase and
surface chemistry that provides engineers ultra-fast access to reliable answers that save time and
money in the Development process. Reaction Design’s FORTÉ is an advanced Computational Fluid
Dynamics (CFD) simulation package for realistic 3D modeling of fuel effects in internal combustion
engines with superior Time-to-Solution metrics that fit in commercial development timeframes. Reaction
Design’s ENERGICO software brings accurate chemistry simulation to gas turbine and boiler/furnace
combustion systems using automated reactor network analysis. Reaction Design also offers
the CHEMKIN-CFD software module, which brings detailed kinetics modeling to other engineering
applications, such as CFD packages. Reaction Design’s world-class engineers, chemists and
programmers have expertise that spans multi-scale engineering from the molecule to the production
plant. Reaction Design serves more than 400 customers in the commercial, government and academic
markets.
FORTÉ-APP-HCCI Engine (v1.0) October 13, 2011 www.reactiondesign.com 9
1-858-550-1920

10. HCCI Engine Performance Evaluation using
FORTÉ Simulation with Detailed Chemistry
Reaction Design can be found online at www.reactiondesign.com.
CHEMKIN®, CHEMKIN-PRO® and Reaction Design® are registered trademarks of Reaction Design.
CHEMKIN-CFD and Model Fuels Consortium are trademarks of Reaction Design.
FORTÉ-APP-HCCI Engine (v1.0) October 13, 2011 www.reactiondesign.com 10
1-858-550-1920