To be successful in the fi eld of automotive engine design, you have to stay

ahead—not only of the competition but also of new emissions mandates.

Computational fl uid dynamics (CFD) can help engine designers create

higher performance, lower emissions internal combustion engines without

costly physical prototyping. CFD lets engine designers visualize and test

fuel and ignition behaviors within a virtual combustion chamber, providing

a faster way to design cleaner, more effi cient engines by simulating ignition

and fuel dynamics.

But combustion CFD can only be of value to engine designers if the model-

ing results predict real-life engine behaviors. For simulations to predict ac-

tual performance and pollutant emissions for internal combustion engines,

you need simulation tools that accurately account for the chemical kinetics

of the combustion process. Simulation solutions that rely on drastically sim-

plifi ed fuel chemistries and ever-fi ner meshing technologies fall far short of

this goal.

Below we discuss four essential facts about the role of accurate chemistry

in modern engine design. Our advice? Follow the chemistry.

Essential Fact #1. Accurate chemistry is crucial for predictive internal

combustion CFD.

Combustion CFD is a very complex, computation-intensive technology.

When running multi-stage simulations with thousands of variables in tra-

ditional CFD solutions, computation times can easily stretch into days. As

White Paper

Four Essential Facts about Chemistry and Combustion CFD

A comprehensive understanding of combustion processes is now possible using computa-

tional fl uid dynamics (CFD), but only if the underlying chemistry of combustion is taken fully

into account. With more than 60 fuel components reacting via complex chemical mechanisms

in short time intervals, knowledge of which components react in which order is essential to

successful, robust combustion modeling. Recognizing this, ANSYS has incorporated the most

comprehensive, validated fuels chemistry database ever assembled — The Model Fuels Library

— in the ANSYS Forte combustion CFD solver to design more cost-eff ective, effi cient, cleaner

burning engines.

In addition to proving that chemistry really does matter, ANSYS Forte also reveals other

important combustion facts: combustion modeling can be both fast and accurate by using

sparse-matrix technologies; soot formation can be modeled by following soot precursors

through the nucleation, growth, agglomeration and oxidation stages; and engine knock can be

simulated using advanced, proven mathematical techniques and algorithms.

1

Four Essential Facts about Chemistry and Combustion CFD

simulations are successively tweaked to test incremental changes to engine

geometries and fuel models, consecutive runs of simulations can take

weeks or months before an optimized design is realized.

To speed design time, many CFD solutions incorporate simplifi ed represen-

tations of combustion chemistry in their simulations, trusting that severe

mesh refi nements can make up in computational detail what they lack in

precise chemistry. These simplifi ed fuel models rely on weakly validated,

third-party mechanisms from disparate and incompatible sources. Using

models from multiple sources makes it very diffi cult to blend or customize

fuels in simulations because species and reactions may be duplicated —

perhaps in a contradictory way — in diff erent sources. Models with these

drawbacks may have been “good enough” when emissions regulations were

less stringent, particularly when the models were later calibrated to include

data derived from empirical methods. But what was good enough back then

is no longer suffi cient.

Better models are needed now because motor fuels have become more

complex. Not only do they vary by seasonal formulation (for instance, in the

U.S. summer-grade gasoline contains less butane than winter formulations),

but fuels also diff er by region and by application: Diesel fuel used in the

U.S. has diff erent properties than diesel fuel used in Europe. We also now

have alternative fuels such as ethanol and bio-diesel in addition to petro-

leum-derived hydrocarbon fuels. To understand the eff ects of these diverse

fuel types, as well as of blended, multi-component fuels, chemically correct

fuel models are required.

ANSYS Forte meets these requirements by incorporating the proven Model

Fuels Library technology into its design fl ow to account for these diverse

fuel types. The Model Fuels Library — the most comprehensive fuels chem-

istry database ever assembled —was the brainchild of Reaction Design, now

part of ANSYS. In 2005, the company assembled the Model Fuels Consor-

tium, a 20-member group that includes global leaders in energy and engine

manufacturing such as GM, Toyota, and VW. The consortium investigated

more timely and cost-eff ective design of cleaner burning, more effi cient

engines and fuels through the use of chemically accurate fuel component

models in software simulation and modeling. Reaction Design collected

and validated the consortium’s fuel data into the Model Fuels Library, a

database of detailed chemical mechanisms for over 60 fuel components,

representing every class of reaction important for combustion simulations.

Today, the Model Fuels Library is at the heart of ANSYS Forte, giving it the

power to perform highly accurate combustion CFD simulations. Forte is fast

and easy to use. Its detailed kinetics and fuel composition models can cap-

ture all aspects of modern engine behavior, including fuel ignition, fl ame

propagation, pollutant emissions, particulate formation and engine knock-

ing, as well as the eff ects of fuel variability and multi-fuel strategies.

ANSYS Forte’s ability to calculate combustion with accurate chemistry

reduces reliance on costly calibration, speeding design and innovation to

match commercial development time frames.

2

Spray droplet visualization at the beginning of injector for a SI
sector-mesh simulation

Visualization of velocity magnitude with velocity vectors on a cut
plane through an intake valve centerline

Essential Fact #2. Accurate chemistry modeling is fast:

You can have speed and accuracy.

It’s true that many CFD packages can’t handle the computational challenge

presented by accurate fuel models. The algorithms that represent precise

chemical mechanisms in fuel models are indeed complex and can add to

compute time when they are used together to represent multicomponent

fuels.

But ANSYS Forte has unique algorithms that address this challenge. The

“accuracy versus compute time” compromise is eliminated by building on

sparse-matrix technologies, the gold standard for chemical kinetics simula-

tion. The solver in ANSYS Forte performs Dynamic Cell Clustering, which

groups cells that have similar kinetic conditions at each time-step to elimi-

nate duplicate calculations. It also incorporates Dynamic Adaptive Chemis-

try, which automatically reduces the kinetics on-the-fl y at every time-step;

using only the necessary chemistry at a given time leads to huge savings in

solution time, with no loss in accuracy.

This changes the well-established scaling equation: instead of scaling with

the cube of the number of species, as is normally the case, the simulation

time scales linearly. So you can include as many reactions as you require

for accurate simulations, without incurring a compute time penalty. You

can incorporate larger, more accurate fuel models into your simulation and

achieve compute times comparable to those with severely reduced, less

accurate models.

Essential Fact #3. Soot formation can be simulated with CFD

In the past decade, engine exhaust regulations have been focused on limit-

ing the total amount of soot and NOx emissions. Medical studies show that

soot particles smaller than 100 nanometers found in combustion engine

exhaust can be especially harmful to human health, and are responsible for

illnesses ranging from asthma to certain types of cancer. Future particulate

matter (PM) regulations, such as the European Union’s extended Euro 6

standard and the California Air Resources Board’s proposed Ambient Air

Quality Standards, restrict soot particle size and numbers, in addition to

total emissions mass limits.

Such requirements are particularly challenging for engine designers. While

the industry is working to meet these higher standards, conventional

combustion engine design typically requires several multi-year cycles of

building and testing prototype units.

Soot phenomena are notoriously diffi cult to simulate and too complex to run

in conventional CFD software, due to the physics and chemical reactions

leading up to soot formation. But ANSYS Forte can predict soot particle

sizes and track their progress in an engine simulation. The software’s

Four Essential Facts about Chemistry and Combustion CFD

3

ANSYS Forte delivers dramatic reductions in time-to-solution
over conventional CFD approaches

Soot formation process simulated with ANSYS Forte

Four Essential Facts about Chemistry and Combustion CFD

4

chemical models follow the reactions from multiple soot precursors through

the nucleation, growth, agglomeration and oxidation of particles to predict

particle size distributions exiting the engine.

Using ANSYS Forte simulations to explore fuel variables, design engineers

can create fuel models and engine geometries that minimize production of

soot-related reactions.

Essential Fact #4. Engine knock can be simulated with CFD

Knocking occurs when the highly compressed fuel and air mixture in the

combustion chamber auto-ignites, either before or after the spark that is

meant to trigger ignition. The spark starts out as an ignition kernel, then

quickly transitions into a thin fl ame surface that moves across the chamber,

sometimes leaving unburned gas in its wake. When this unburned fuel ig-

nites outside of the planned spark event, it results in knocking. Knock limits

engine performance and over time can severely damage the engine.

Accurately modeling the location and structure of the fl ame front as it

expands into the combustion chamber is extremely important for predict-

ing knock. But simulating auto-ignition within the combustion chamber is

very diffi cult with conventional CFD approaches that rely on mesh refi ne-

ment and inaccurate chemistry. Since the scale of the fl ame front thickness

is signifi cantly smaller than computational mesh—even with severe grid

refi nement—CFD simulations that rely on mesh to resolve the fl ame loca-

tion will require an inordinately large number of tiny cells to resolve the

fl ame topology suffi ciently. Meshes with such excessively large numbers of

small computational cells can easily get bogged down by the tiny time steps

needed to maintain simulation stability, and require an impractical amount

of computation time.

Rather than compute-heavy approaches that rely on extreme mesh refi ne-

ments, ANSYS Forte uses proven mathematical techniques and algorithms,

such as the Discrete Particle Ignition Kernel (DPIK) model and the G-equa-

tion fl ame-front tracking technique, to provide fast and accurate resolutions

of fl ame propagation. When coupled with direct use of detailed chemical

kinetics from the Model Fuels Library and Forte’s knock-index calculator,

ANSYS Forte helps ensure that engine designers have accurate simulations

for predicting auto-ignition and knock propensity in the engine under a

range of operating conditions.

ANSYS Forte tracks the progress of the fl ame front across the combustion chamber

Summary

Chemistry is crucial. It’s at the heart of combustion, and for internal

combustion CFD to accurately predict real-world engine behavior it must

precisely account for real chemical kinetics.

ANSYS Forte provides engine designers with tools to create combustion CFD

simulations that can quickly and accurately predict emissions that translate

reliably to actual engine designs—with far less trial and error hardware

prototyping. Its realistic 3D modeling capability taps complex algorithms

to describe the physics and chemistry of combustion and accurately predict

real-life fuel eff ects. Fuel components derived from the industry-validated

Model Fuel Library enable ANSYS Forte to simulate combustion for a large

variety of new or existing fuel blends and foresee what emissions will occur

for a wide range of operating conditions.

By using accurate fuel models based on precise chemistry, ANSYS Forte

greatly increases the predictive quality of combustion simulations, help-

ing engine designers to more quickly and eff ectively meet strict regulatory

guidelines and create advanced clean engine and fuel technologies.

Four Essential Facts about Chemistry and Combustion CFD

ANSYS, Inc. is one of the world’s leading engineering simulation software provid-

ers. Its technology has enabled customers to predict with accuracy that their prod-

uct designs will thrive in the real world. The company off ers a common platform of

fully integrated multiphysics software tools designed to optimize product develop-

ment processes for a wide range of industries, including aerospace, automotive,

civil engineering, consumer products, chemical process, electronics, environ-

mental, healthcare, marine, power, sports and others. Applied to design concept,

fi nal-stage testing, validation and trouble-shooting existing designs, software from

ANSYS can signifi cantly speed design and development times, reduce costs, and

provide insight and understanding into product and process performance. Visit

www.ansys.com for more information.

Toll Free U.S.A./Canada:

1.866.267.9724

Toll Free Mexico:

001.866.267.9724

Europe:

44.870.010.4456

[e-mail]

ANSYS, Inc.

Southpointe

275 Technology Drive

Canonsburg, PA 15317

U.S.A.

724.746.3304

[e-mail]

© 2015 ANSYS, Inc. All Rights Reserved.