CFD Combustion Models for IC Engines

Rolf D. Reitz

Engine Research Center

University of Wisconsin-Madison

http://www.cae.wisc.edu/~reitz

ERC Symposium, June 7, 2007

1. Characteristic Time Combustion Model (KIVA-CTC)

• Auto-ignition model (SHELL) – ignition with 8 reaction steps

• Combustion model (CTC) – turbulence mixing timescales

2. Representative Interactive Flamelet Model (KIVA-RIF)

• Flamelet equations/detailed chemistry coupled with CFD (Peters et al.)

• Effect of turbulence is modeled with average scalar dissipation rate

3. Direct Integration of CFD with Chemistry (KIVA-CHEMKIN)

• Each computational cell treated as well stirred reactor

• ERC reduced mechanism for n-heptane – chemistry timescales

4. KIVA-CHEMKIN-G Model

• Explicit modeling of turbulent flame propagation combined with detailed

chemistry

Turbulence vs. chemistry time scalesTurbulence vs. chemistry time scales

Combustion and Emission Models at the ERC

Eddy breakup

timescales

Chemistry

timescales

Auto-ignition and Detailed Chemistry Models

Gasoline

Diesel

Bio-diesel

Aromatic

EhthanolAlcohol

25 species

51 reactions

30 species

65 reactions

41 species

150 reactions

Comprehensive

mechanism

Skeletal

mechanism

Multi-fuel

Mechanism

(ERC-PRF-Bio)

Skeletal Chemistry Mechanism for Multi-Surrogate Fuels

Multi-component fuelsurrogate fuel

857 species

3586 reactions

570 species

2580 reactions

264 species

1219 reactions

Iso-octane

N-heptane

Methyl

butanoate

Toluene

Dr. Youngchul Ra

Jessica Brakora

SAE 2004-01-0558,

SAE 2007-01-0190,

SAE 2007-01-0165

47

41

Shock tube measurements

(Fieweger et al. 1997)

Calculation using ERC-PRF

mechanism

Validation - Auto-ignition Model

n-heptane-iso-octane/air mixtures, stoichiometric, P=40 Bar)

SAE 2007-01-01650.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5

0.01

0.1

1

10

100

Ignitio

n D

ela

y T

ime (

ms)

1000(K)/T

100% i-octane 90% i-octane / 10% n-heptane 80% i-octane / 20% n-heptane 60% i-octane / 40% n-heptane 100% n-heptane

Validation – Cat HD engine

0

20

40

60

80

100

120

140

160

180

0.135 0.20 0.23 0.27 0.29 0.39

CO

(g

/Kg

f)

Equivalence ratio

Experiment Shell-CTC Chemkin

-40 -20 0 20 40

0

1

2

3

4

5

6

0

100

200

300

400

500

600

700

800

900 Experiment Shell-CTC CHEMKIN

P

ressure

(M

Pa)

CAD ATDC

AH

RR

(J/d

eg)

rpm = 1737eq_ratio = 0.23 c_ratio = 16.1inj_timing = -113 ca

-40 -20 0 20 40

0

1

2

3

4

5

6

0

100

200

300

400

500

600

700

800

900700 rpmeq_ratio = 0.27c_ratio = 16.1inj_timing = - 274.5 ca

Pre

ssure

(M

Pa)

CAD ATDC

Experiment Shell-CTC CHEMKIN

AH

RR

(J/d

eg)

SAE 2002-01-0418, SAE 2007-01-0190

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0.135 0.20 0.23 0.27 0.29 0.39

NO

x (

g/K

gf)

Equivalence ratio

Experiment Shell-CTC Chemkin

Gasoline HCCI

Validation - GM Light-duty Diesel Engine

GM 1.9L 4-cylinder

Bore Stroke 82 90.4 mm

Compression Ratio 16.6: 1

Engine Speed 2000 rev/min

IMEP 5.5 Bar

EGR 67% PCCI

SAE 2007-01-0193

SOIC = –39, -32 and –23

SOIC=-32

KIVA-CHEMKIN

Advances in SI Engine Combustion Modeling

G-equation Combustion Model with Detailed Chemical Kinetics

SAE 2007-01-0165

GkDGSGvvt

GTT

uvertexf

~~~~)(

~0

=+

End Gas

G-Equation

Flame

propagationBurnt Gas

Discrete particle

ignition model

SAE 2000-01-2809

SAE 2003-01-0722

Auto-ignition

(detailed

kinetics)

-100 -50 0 50 1000.0

0.5

1.0

1.5

2.0

2.5

PFI mode

-32 OATDC

Pre

ssu

re (

MP

a)

Crank Angle (oATDC)

EXPT SIMU

-100 -50 0 50 1000.0

0.5

1.0

1.5

2.0

2.5

Pre

ssu

re (

MP

a)

Crank Angle (oATDC)

EXPT SIMU

PFI mode

-44 OATDC

PFI Spark

sweep

Ford DISI

Bore Stroke 89 79.5 mm

Compression Ratio 12 : 1

Engine Speed 1500 rev/min

DI Spark

sweep

-80 -60 -40 -20 0 20 40 60 80 100-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0

5

10

15

20

25

Pre

ssu

re (

MP

a)

Crank Angle (oATDC)

EXPT SIMU

Heat

Rele

ase R

ate

(J/D

eg

Spk = -20 oATDC

-80 -60 -40 -20 0 20 40 60 80 100-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

0

5

10

15

20

25

Pre

ssu

re (

MP

a)

Crank Angle (oATDC)

EXPT SIMU

Spk = -32 oATDC

Heat

Rele

ase R

ate

(J/D

eg

-80 -60 -40 -20 0 20 40 60 80 100-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

0

5

10

15

20

25

30

35

Pre

ssu

re (

MP

a)

Crank Angle (oATDC)

EXPT SIMU

Heat

Rele

ase R

ate

(J/D

eg

MAP = 75 kPa

-80 -60 -40 -20 0 20 40 60 80 100-0.5

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

0

5

10

15

20

25

30

35

Pre

ssu

re (

MP

a)

Crank Angle (oATDC)

EXPT SIMU

Heat

Rele

ase R

ate

(J/D

e

MAP = 100 kPa

DI MAP

sweep

SAE 2007-01-0165

Validation – Ford SI EngineKIVA-CHEMKIN-G

-15 -10 -5 0 5 10 15 20 25

0

200

400

600

800

1000

1200

Injection Profile

Pre

ssu

re (

MP

a)

AH

RR

(J

/de

g)

CAD ATDC

-1

0

1

2

3

4

5

6

7

8

9High-T, Long-ID TEST

CHEMKIN G_Model

O2 = 21%

SOI = -5

Experiment KIVA-CHEMKIN KIVA-CHEMKIN-G

Validation - Sandia Cummins Diesel

High-T, long-ID Condition (Premixed Combustion)

SAE 2006-01-0055

Green: OH

Red: Soot

Experimental

-20 -15 -10 -5 0 5 10 15 20 25 30 35 40

0

300

600

900

1200

1500

1800 Case 1 Case 2 Case 3 Case 4 Case 5

Experimental Measurements

AH

RR

(J

/de

g)

CAD ATDC

-2

0

2

4

6

8

10

12

14

16

Pre

ssu

re (

MP

a)

KIVA-CHEMKIN-G

-20 -15 -10 -5 0 5 10 15 20 25 30 35 40

0

300

600

900

1200

1500

1800 Case 1 Case 2 Case 3 Case 4 Case 5

Model Predictions

AH

RR

(J

/de

g)

CAD ATDC

-2

0

2

4

6

8

10

12

14

16

Pre

ssu

re (

MP

a)

Validation - Cummins Dual-Fuel Engine

Case1 Case2 Case3 Case4 Case50.000

0.005

0.010

0.015

0.020

0.025

0.030

NOx

Em

iss

ion

s (

gra

ms

)

C

Measured

Predicted

NOx

SAE 2007-01-0171

Case 1 2 3 4 5

TIVC (K) 392 397.5 403 405 408

BMEP 10 bar

SOI -87 BTDC

PIVC 2.1 bar

NG 0.44

CR 14.5

Primary Fuel:

Natural Gas

Ignition Source:

Diesel Pilot (10%)

KIVA-CHEMKIN vs KIVA-CHEMKIN-G

-50 -40 -30 -20 -10 0 10 20 30 40 50

0

300

600

900

1200

1500

1800 Test KIVA-CHEMKIN

AH

RR

(J/d

eg

)

CAD ATDC

-2

0

2

4

6

8

10

12

14

16Case 1

Pre

ssu

re (

MP

a)

-50 -40 -30 -20 -10 0 10 20 30 40 50

0

300

600

900

1200

1500

1800 Test KIVA-CHEMKIN-G

AH

RR

(J/d

eg

)

CAD ATDC

-2

0

2

4

6

8

10

12

14

16Case 1

Pre

ssu

re (

MP

a)

Without flame-

propagation

combustion is

too slow

Validation - Cummins Dual-Fuel Engine

Summary and Conclusions

• Current CFD models describe important physical and chemical

processes of engine flows, but validation experiments are needed

for model development and improvement

• CFD combustion models capture engine performance and

emissions trends and can be combined with optimization tools for

design and evaluation of engine design concepts

• Methodologies are in place for modeling a wide range of

combustion regimes:

HCCI PCCI Diesel SI flame propagation dual-fuel

• Multi-component fuel vaporization models can be coupled with

advanced chemical kinetics models

• Surrogate fuel models are under development to represent

representative practical fuels (MB, toluene, ethanol…..)

Multi-Component fuel modeling – Dr. Youngchul Ra

Diesel

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

0

10

20

30

40

50

0 50 100 150 200 250 300

gasoline composition

iso-octane approximation

distribution or mole

distribution or mol

molecular weight

Continuous fp(I)

Discrete gp(mwi)

Single comp approx

6.393-C/H ratio

0.3%Olefins

42%Napthenes

42%Paraffins

7.3ppmSulfur

16%Aromatics

Gasoline* * Adapted from J. Farrel, Exxon Mobil Corp.

PRF50: Emissions

0.00

0.05

0.10

0.15

0.20

0.25

-50 -45 -40 -35 -30 -25 -20

start of injection command [deg ATDC]

NO

x [

g/k

g-f

]

PRF50

Diesel-sgl

NOxLTC regime (EGR=65%)

Injection sweep

0

20

40

60

80

100

120

140

-50 -45 -40 -35 -30 -25 -20

start of injection command [deg ATDC]

HC

at

EV

O [

g.k

g-f

]

PRF50

Diesel-sgl

HC0

100

200

300

400

500

600

700

800

-50 -45 -40 -35 -30 -25 -20

start of injection command [deg ATDC]

CO

at

EV

O [

g/k

g-f

]

PRF50

Diesel-sgl

CO

Reduced Biodiesel Mechanism – Jessica Brakora

Mechanism Reduction Method– SENKIN constant volume analysis

– Identify key species and reaction pathsbased on ignition timing

• Peak concentration: remove species thatnever exceed specified mole fractionthresholds (-112 species)

• XSENKPLOT reaction flux analysis:identify key reaction pathways, removeless important paths (-111 species)

– Adjust rate constants for key reactionsto optimize ignition timing

XSENKPLOT reaction paths: MB fuel breakup

Large flux to “mb2j” (C5H9O2);test removal of other paths

LLNL methyl butanoate (MB)*

– Surrogate to represent the large methylesters in biodiesel

– MB chemical formula: C5H10O2

– 264 species, 1219 reactions

*Proc. Comb. Inst., Vol 28, 2000/pp.1579-1586

1.E-05

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

600 700 800 900 1000 1100 1200 1300 1400

Initial Temperature [K]

Ign

itio

n D

ela

y [

s]

LLNL detailed (264 species)

rm spec < 10 -̂10 (199 spec)

rm spec < 10 -̂9 (179 spec)

rm spec < 10 -̂8 (162 spec)

No noticeable change in ignition;these species can be removed

Reduced MB Kinetic Mechanism

P = 40bar, Phi = 1.0

1.E-02

1.E-01

1.E+00

1.E+01

1.E+02

600 700 800 900 1000 1100 1200 1300 1400

Initial Temperature [K]

Ign

itio

n D

ela

y [

ms

]

LLNL detailed

MB reduced

mb

mb2ooh mb2oo mb2j

ch3o2h (ch3o, ch2o, ch3o2)

mb2o

c2h5chome2*o

me2j*o

(ch2co,ch3oco)

(ch3oco) mp2d

ch2cho

ch2co(c2h5)

(ch2o)

mb2ooh4j

mb2ooh4oo

mb4ooh2*o

mp3j2*o

(ch3oco, ch2co)

These species further break down into

lower-level carbon species

MB reduced coupled with ERC n-heptane: 41 species, 150 reactions

ERC n-heptane mechanism SAE 2004-01-0558: 34 species

ERC PRF Reduced Chemistry SAE 2007-01-0190, SAE 2007-01-0165: 47 species

Auto-ignition and Detailed Chemistry Models

OH

RH + O2 ROOH

ORO

First Stage Ignition

Olefin

RH + O2

HO2 H2O2

H2O2 + (M) OH + OH +

(M)

Second Stage Ignition

Diesel Gasoline

Multi-component Fuel – DMC/PRF Models

0

2

4

6

8

10

12

14

-30 -20 -10 0 10 20 30 40 50

CA [deg ATDC]

pre

ss

ure

[M

Pa

]

0

40

80

120

160

200

240

280

HR

R [

J/d

eg

]

P, inj= -45

P, inj= -42

P, inj= -39

P, inj= -33

P, inj= -27

P, inj= -21

P, iC8

P, nC7

HRR, inj= -45

HRR, inj= -42

HRR, inj= -39

HRR, inj= -33

HRR, inj= -27

HRR, inj= -21

HRR, iC8

HRR, nC7

iso-octane

n-heptane

PRF50 fuel