Reciprocating Internal Combustion Engines Lecture... · prior written permission of the owner, Rolf D. Reitz. Part 3: Chemical Kinetics, HCCI & SI Combustion . Short course outline:

Reciprocating Internal Combustion Engines

Prof. Rolf D. Reitz

Engine Research Center

University of Wisconsin-Madison

2014 Princeton-CEFRC

Summer School on Combustion

Course Length: 15 hrs

(Mon.- Fri., June 23 – 27, 2014)

1 CEFRC2-3, 2014

Copyright ©2014 by Rolf D. Reitz.

This material is not to be sold, reproduced or distributed without

prior written permission of the owner, Rolf D. Reitz.

Part 3: Chemical Kinetics, HCCI & SI Combustion

Short course outline:

Engine fundamentals and performance metrics, computer modeling supported

by in-depth understanding of fundamental engine processes and detailed

experiments in engine design optimization.

Day 1 (Engine fundamentals)

Part 1: IC Engine Review, 0, 1 and 3-D modeling

Part 2: Turbochargers, Engine Performance Metrics

Day 2 (Combustion Modeling)


Part 4: Heat transfer, NOx and Soot Emissions

Day 3 (Spray Modeling)

Part 5: Atomization, Drop Breakup/Coalescence

Part 6: Drop Drag/Wall Impinge/Vaporization/Sprays

Day 4 (Engine Optimization)

Part 7: Diesel combustion and SI knock modeling

Part 8: Optimization and Low Temperature Combustion

Day 5 (Applications and the Future)

Part 9: Fuels, After-treatment and Controls

Part 10: Vehicle Applications, Future of IC Engines


2 CEFRC2-3, 2014

Modes of engine combustion http://www.erc.wisc.edu/combustion.php

HCCI uses a hybrid combustion strategy. Premixed fuel and air is inducted,

but instead of igniting with a spark as in a SI engine, the high temperature from

compression causes the mixture to spontaneously react, like in a diesel engine.

Ignition occurs at slightly different times at different locations in the chamber.

One feature of HCCI combustion is how quickly the fuel is consumed.

3 CEFRC2-3, 2014


4 CEFRC2-3, 2014

Chemistry: importance of fuels

Part 3: Chemical Kinetics, HCCI & SI Combustion Curtis, 2014

5 CEFRC2-3, 2014

New advanced combustion regimes

HCCI

Part 3: Chemical Kinetics, HCCI & SI Combustion Daw, 2013

Basic combustion concepts – Spark Ignition (SI)

Characteristic Time Combustion (CTC) model

How can SI engines operate with engine speeds from 100 to 20,000 rev/min?

Because turbulent flame speed, ST, scales with rpm!

ST

tc ~ k/e ~ Lpiston / Vpiston

Kinetic energy, k~Vpiston2

Integral length scale lI ~ Lpiston

Kinetic energy dissipation rate,

e ~ Vpiston3/Lpiston

Species conversion rate (Yi, species mass fraction, * local equilibrium solution)

Diffusivity, D ~ k2/e ~ Vpiston Lpiston

Turbulence!

;

fuel/air

burned

Mallard-Le Chatelier propagating wave speed: ~ Vpiston

T

x

iT

dYS D

dtGlassman, 1996

Reitz & Bracco, 1983; Abraham, 1985

6 CEFRC2-3, 2014


Basic combustion concepts – Diesel (CI)

Shell Ignition Model

RH +O2 2R*

R* R* + P + Heat

R* R* + B

R* R* + Q

R* + Q R* + B

B 2R*

R* termination

2R* termination

Af04

R*

B

Q

Switch to Characteristic Time

Combustion model

Ignition

Delay

tc ~ k/e ~ Lnozzle / Vnozzle

Turbulence generated by fuel injection

Kong, 1992

Halstead, 1977

7 CEFRC2-3, 2014


8 CEFRC2-3, 2014

Turbulent mixing

ST

Hot products with

Cold reactants Spark-ignition

t ~ k/e

~ Lpiston / Vpiston

High turbulence

- faster combustion

burned unburned

ST/S

L

Matalon, 2011


Diesel

fuel

air

air

Delayed ignition (PCCI)

- better mixing ST=0

t~ k/e ~ Lnozzle / Vnozzle

Injected fuel with

entrained air

• Diesel engine with spray (diffusion) combustion:

Rich mixtures (soot) & high temperatures (NOx) higher TE ~45%

• Gasoline engine spark-ignition with flame propagation:

High turbulence for high flame speed heat losses. Issues: NOx and UHC/CO,

knock (CR, fuels), throttling losses low thermal efficiency TE ~25%

spark-ignition

• H/Premixed Charge Compression Ignition – LTC, chemistry controlled (CR):

Sensitive to fuel, poor combustion/load control, low NOx-soot TE ~50%

H/PCCI diesel

Summary of combustion regimes

9 CEFRC2-3, 2014


10 CEFRC2-3, 2014

Premixed volumetric combustion & chemical kinetics

Species and energy conservation equations

Constant volume combustion – Well-Stirred-Reactor (WSR)

;

nr reactions

ns species

chemical label

mass fraction

molecular weight

species energy

reactant/product

stoichiometric

coefficients

I specific internal energy

( ) [ ( )] c si ii i iu D

t

Williams, 1988

'' '

, ,

1

( ) ( , ), 1,...,rn

i ik i k i k s

k

dY WT i n

dt

Y

' '', ,

, ,

1 1

( , )

k i k is sn n

i ik f k b k

i ii i

Y YT

W W

Y

1

( )1( , ) ( , )

( , )

sn

i i

iv i

e T dYdTT T

dt c T W dt

Y Y

Y

ei

ei


Homogeneous charge: no spatial gradients

11 CEFRC2-3, 2014

/ii

Y

t

Consider single overall reaction

0

,

1

snf i i

i p

hT

t c

4 3 2

kCH OH CH H O

exp( / )bk AT E RT

71.6 10 ( , , ), 1.83, 11.6( / )A cm mol s b E kJ mol

4

4

44

[ ][ ][ ]

CH

CH

d CHk CH OH

dt W

0

2 1012

4 1012

6 1012

8 1012

1 1013

1.2 1013

1.4 1013

0

0.5

1

1.5

2

0 500 1000 1500 2000 2500

T

k

k

CH4

Part 3: Chemical Kinetics, HCCI & SI Combustion Law, 2006

0

0.1

0.2

0.3

0.4

0.5

0.6

0 0.2 0.4 0.6 0.8 1

F(U

)=U

m+

1(1

-U)m

U

HCCI: Ignition delay

bt

U

1.0

Uo

Ignition

delay

12 CEFRC2-3, 2014

/ii

Y

t

1( ) (1 )m mdUF U U U

dtb

Consider single component system

unburned

burned unburned

T TU

T T

Example:

For U Uo 0

1 1m m

m tU U

b :

So, time to reach, say, 5U0 :

Ignition delay:

0

4

5 mt

mUb

2 1

1. ( , ;(1 ); )

mm t Const F m m m U

Ub

m=0.25

m=0.5

m=1.0

( )F U

U

Cold boundary

difficulty

0

,

1

snf i i

i p

hT

t c

Part 3: Chemical Kinetics, HCCI & SI Combustion Reitz, 1981

Combustion chemistry models – CH4 (15 spec, 31 react.)

Hydrogen-Oxygen Chain

1 H + O2 OH +O

2 H2 + O OH + H

3 H2 + OH H2O + H

4 H2O + O 2 OH

Hydroperoxyl Formation and Consumption

5b H + O2 + M HO2 + M

6 HO2 + H 2 OH

7 HO2 + H H2 + O2

8 HO2 + H H2O + O

9 HO2 + OH O2 + H2O

Conversion of Carbon Monoxide to Carbon Dioxide

10 CO + OH CO2 + H

Methane Consumption

11 CH4 + H H2 + CH3

12 CH4 + OH H2O + CH3

Methyl Reactions

13 CH3 + O CH2O + H

14 CH3+OH CH2O + H +H

15 CH3+OH CH2O + H2

16c CH3 + H CH4

23 CH3 + H CH2 + H2

28 CH3 + OH CH2 + H2O

Formaldehyde Reactions

17 CH2O + H CHO + H2

18 CH2O + OH CHO + H2O

Formyl Reactions

19 CHO + H CO + H2

20 CHO + OH CO + H2O

21 CHO + O2 CO + HO2

22 CHO + M CO + H + M

Methylene Reactions

24 CH2 + O2 CO2 + H2

25 CH2 + O2 CO + OH + H

26 CH2 +H CH + H2

29 CH2 + OH CH2O + H

30 CH2 + OH CH + H2O

Methylidyne Reactions

27 CH + O2 CHO + O

31 CH + OH CH2O + H

CH4+ 2 O2 = CO2 + 2 H2O

H2 O

2 c

hem

istr

y

Conversion to products by sequential fragmentation by H abstraction

Hig

h t

em

pera

ture

End

Initiation

H atom abstraction Start

13 CEFRC2-3, 2014

Part 3: Chemical Kinetics, HCCI & SI Combustion Warnatz, 2006

Chemical kinetic mechanisms for engine simulations

Requirements for mechanisms for practical engine simulations:

• Size can not be too large due to CPU time limitation ~ 100 species

• Capable of predicting auto-ignition delay time accurately

• Contain proper reactions for pollutant formation precursors

Biodiesel surrogates

- Significant mechanism

reduction is required.

Soy biodiesel - Methyl:

- palmitate (C16:0)

- stearate (C18:0)

- oleate (C18:1)

- linoleate (C18:2)

- linolenate (C18:3)

C4 C9 C2

14 CEFRC2-3, 2014

CH4

Part 3: Chemical Kinetics, HCCI & SI Combustion Lu, 2009

Brakora, 2013

0

200

400

600

800

1000

1200

1400

555 580 605 630 655 680 705 730

Initial Temperature (K)

Ind

ucti

on

Peri

od

(s)

Acceleration by

Q•OOH branching

First Stage Ignition

Isomerization steps

O2

Hydrocarbon kinetics - NTC

Second Stage Ignition

H2O2 = OH + OH

15 CEFRC2-3, 2014

.


Fuel: RH R

+OH, H,…

ROO QOOH

Internal H atom

abstraction

b-scission

Smaller R

CH3, C2H5,…

Alkane fuel oxidation

HCO

+

•CH2CH2CH2OOH

n-C3H7• + O2

Isomerization

Chain branching

H + O2 OH + O

Thermal decomposition

H2O2 OH + OH

Blue flame

High

temperature

reaction

CO + OH CO2 + H

1100K

>1200K

Energy release

Chain propagation

QO + OH

OOQOOH

+O2

Internal H atom

abstraction

HOOQ’OOH

Chain branching

HOOQ’O + OH

OQ’O + OH

Cool flames

Negative

temperature

coefficient

Olefin channel

800K

800

~900K

Example: Propane

16 CEFRC2-3, 2014


Typical HCCI Combustion

Temperature and Heat Release Rate

profiles

•

R• H

- RH

+ O2

Degenerate

Branching Path

OO•

OOH •

•OO

OOH

HOO

O

+ O2

- •OH

O

O

•

•OH + +

+ HO2•

+ •OH

+ •OH O

+

O

•

Fast High Temperature

Combustion

T, P

CAD

HR

R

TDC

HCCI combustion kinetics

Aldehydes/ketones

Ethers/

olefins

H2O2

Mehl, 2009

17 CEFRC2-3, 2014


, 7

6

1. n-C7H16 + OH= C7H15-2+ H2O

2. C7ket12 = C5H11CO + CH2O + OH

3. H2O2 + M = OH + OH + M

4. HO2 + HO2 = H2O2 + O2

5. CH4 + HO2= CH3 + H2O2

6. CO + OH= CO2 + H

7. C7H15O2 + O2= C7ket12 + OH

, 7

6

1. n-C7H16 + OH= C7H15-2+ H2O

2. C7ket12 = C5H11CO + CH2O + OH

3. H2O2 + M = OH + OH + M

4. HO2 + HO2 = H2O2 + O2

5. CH4 + HO2= CH3 + H2O2

6. CO + OH= CO2 + H

7. C7H15O2 + O2= C7ket12 + OH

Mechanism reduction – identify key reaction steps

Time

Tem

per

atu

re

Patel, 2004

ERC n-heptane mechanism

18 CEFRC2-3, 2014

3,4,5

6

2

1,7

1st

stage

2nd

stage

Burnout

stage


First stage (t1), main ignition (t2=tig) delay

0.01

0.1

1

10

0.8 0.9 1 1.1 1.2 1.3 1.4 1.5

1000/initial temperature [1/K]

ign

itio

n d

ela

y [

ms

]

Cal, tig

Exp, tig

Cal, t1

Exp, t1

Predicted ignition delay times

validated against shock tube tests

(data from Fieweger)

=1.0 and P=40 bar n-heptane/air

Expts: Fieweger, 1997

Reduced mechanisms: match shock tube and RCM data

Ra, 2008

19 CEFRC2-3, 2014


Mechanism reduction methodology

Reduction of reaction pathways and species number

- combination of chemical lumping, graphical

reaction flow analysis and elimination methods

Reaction rate optimization

- ignition delay curve sensitivity analysis

-5

0

5

10

15

20

25

650 750 850 950 1050 1150 1250 1350 1450

initial temperature [K]

ign

itio

n s

en

sit

ivit

y [

%]

-40

-30

-20

-10

0

10

20

30

40

50

60

gra

die

nt

se

ns

itiv

ity

co

eff

icie

nt

ignition delay sensitivity

gradient sensitivity

0.01

0.1

1

10

600 700 800 900 1000 1100 1200 1300 1400

initial temperature [K]

ign

itio

n d

ela

y [

ms

]

baseline

k=2

k=0.5

2tetra-decane: ROO+O =R-keto+OH

1 210 10

10 10 1 2

(log log )( ) 100

log log ( )

k k

ig

base

t tS T

t k k

Ignition delay sensitivity coefficient

1 210 10

10 1 2

log log

( ) 100log ( )

k k

gr

d t d t

dT dTS Tk k

Ignition delay gradient sensitivity coefficient

Positive Sgr: counter-clockwise rotation

Negative Sgr: clockwise rotation

A = k×Abase

A = k×Abase Pre-exponential:

20 CEFRC2-3, 2014

Part 3: Chemical Kinetics, HCCI & SI Combustion Ra, 2011

No Reaction As Bs Cs Ar Br Cr effect

I 2RH H R H O

II 2RH OH R H O ●↓ ●↑ ●C τ1, p2, p3, τ 4

III 2 2 2RH HO R H O ○↓ ○↓ ○↓ τ 1, P2, P3, τ 4

IV 2 2RH O R HO ○↑ ○↓ ● P2, P3

V 2R O ROO

VI-a ROO = QOOH ●↓ ●↓ ○C τ 1, P2

VI-b 2QOOH + O = OOQOOH ○↓ ●↓ P2, P3

VI-c OOQOOH = R-keto + OH ○↓ ●↓ P2, P3

VII 2R-keto = CH O + R'CO + OH ●↓ ○C τ 1

VIII 1 2R'CO = X + X + CO

IX 1 2 3R = S + S + S ●↑ ○ P2, P3, τ 4

Sensitivity of ignition delay curves of n-heptane oxidation

- solid circle, open circle and blank entry denote

dominant, mild and not significant influence, respectively.

- C indicates counter-clockwise rotation.

- Circle only indicates clockwise rotation.

Mechanism reduction – group reaction classes Ignition delay

sensitivity coefficient

Ignition delay gradient

sensitivity coefficient

21 CEFRC2-3, 2014


22 CEFRC2-3, 2014

ERC-MultiChem: PRF

41 species, 158 reactions base mechanism

Source mechanisms: LLNL n-heptane

(560 species; 2,539 reactions), isooctane

(857 species; 3,606 reactions),

ERC n-heptane (29 species; 52 reactions)

0.01

0.1

1

10

0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5

1000/T [1/K]

ign

itio

n d

ela

y [

ms]

PRF

MultiChem

Exp, Fieweger et al. (1997)

0.01

0.1

1

10

100

0.8 0.9 1 1.1 1.2 1.3 1.4 1.5 1.6

1000/initial temperature [1/K]

ign

itio

n d

ela

y [

ms]

iC8

PRF90

PRF80

PRF60

nC7

Exp, iC8

Exp, PRF90

Exp, PRF80

Exp, PRF60

Exp, nC7

0.01

0.1

1

10

100

0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5

1000/T [1/K]

ign

itio

n d

ela

y [

ms

]

PRF

MultiChem


=1.0, iC8H18/air, 40 bar

=1.0, 40 bar

nC7H16/air


Cyclo alkanes

n-alkane

n-alkane

iso-alkanes

Aromatics

cyclohexane

decalin cyclohexane

n-dodecane

n-heptane

n-octadecane

n-tetradecane

naphthalene

mcymene

tetralin

n-pentylbenzene

n-heptylbenzene

toluene

heptamethyl nonane

tetramethyl hexane

iso-octane

Chemical class grouping: “MultiChem” skeletal mechanism

25 species

51 reactions

857 species

3586 reactions

Physical property

surrogates

Chemistry

surrogates LLNL Detailed

mechanism ERC reduced

mechanism*

Mu

ltiC

he

m M

ech

an

ism

10

0 s

pe

cie

s, 3

48

re

actio

ns

23 CEFRC2-3, 2014


Ignition delay validations - “MultiChem”

0.1

1

10

100

1000

0.9 1 1.1 1.2 1.3 1.4 1.5

1000/T [1/K]

ign

itio

n d

ela

y [

ms

]

Experiment

Model

Propane phi=1.0 Pin= 30 bar

Gauthier CNF 2004

0.01

0.1

1

10

0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5

1000/T [1/K]ig

nit

ion

de

lay

[m

s]

Model


nHeptane phi=1.0 Pini=40 bar

1

10

100

1000

10000

0.55 0.6 0.65 0.7 0.75 0.8

1000/T [1/K]

ign

itio

n d

ela

y [

mic

r-s

ec

]

EXP (Bounaceur et al.)

Bounaceur et al.

Andrae et al.

ERC-MultiChem

Toluene

10

100

1000

10000

0.8 0.9 1 1.1 1.2 1.3

1000/T [1/K ]

tig

n [

mic

ro-s

]

E xperiment

Model

MCH Phi=1.0

0.01

0.1

1

10

0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5

1000/T [1/K]ig

nit

ion

de

lay

[m

s]

Experiment

Model

Decalin phi=1.0 Pini=40 bar

0.01

0.1

1

10

100

0.7 0.8 0.9 1 1.1 1.2 1.3 1.4 1.5

1000/T [1/K]

ign

itio

n d

ela

y [

ms

]

Model

Experiment

iC8H18 phi=1.0 Pini=40 bar

Fieweger CNF 1997

Shen,Energy & Fuels, 2009 Bounaceur IJCK 2005; Andrae CNF 2005

Fieweger CNF 1997

8 Surrogate fuels: n-heptane,

iso-octane, tetradecane, cyclohexane,

toluene, decalin, ethanol, MB/D……

24 CEFRC2-3, 2014


3-D CFD modeling

Solve conservation equations on (moving) numerical mesh

Mass

Species

Momentum

Energy

combustion source terms

Amsden, 1997

25 CEFRC2-3, 2014


Sparse analytical Jacobian

formulation

All functions and equations are evaluated in

matrix form

ODE system function, analytical Jacobian

evaluation and linear system solution

achieve linear scaling with ns

non-zeroes: 860; sparsity: 62.7%

ERC PRF mechanism Jacobian structure LLNL n-heptane mech. Jacobian structure

non-zeroes: 3571; sparsity: 86.2% non-zeroes = 49763; sparsity: 99.7%

LLNL MD mechanism Jacobian structure

ns

47 (62.7%) 160 (86.2%) 2878 (99.7%)

101

102

103

104

10-3

10-2

10-1

100

101

102

number of species, ns

tim

e p

er e

va

lua

tio

n [

ms]

SpeedCHEM performance scaling

function, y' = f(y)

Jacobian, J(y) = df(y)/dy

linear system solution

LLNLMD

LLNLPRF

ns

ERCPRF

ERCnC

7H

16

LLNLnC

7H

16ERCmultichem

Sparsity of hydrocarbon fuel mechanisms increases

with size

0

,

1

snf i i

i p

hT

t c

/i

i

Y

t

26 CEFRC2-3, 2014

Perini, 2014

3-D CFD: Improved solver numerics


Promising, efficient approach for practical engine simulations

1.Numerically exact solution (no mechanism reduction or manipulation).

2.Speed-up of more than three orders of magnitude at large sizes (ns>1000)

3.Even for modest sizes (~50-500 species), overall CPU time for chemistry is reduced

by 3-10 times in comparison with dense chemistry integrators

4.Preconditioned Krylov solution for future, very large mechanisms

9 reaction mechanisms tested -ns = 29 to 7171 -nr = 52 to 31669 18 ignition delay calculations per mech - phi = [0.5, 1.0, 2.0] - T0 = [650, 800, 1000] K - p0 = [20, 50] bar - t = [0.1] s

101

102

103

104

10-3

10-2

10-1

100

101

102

103

104

number of species

CP

U t

ime

[s]

SpeedCHEM ignition delay time calculation scaling

Direct dense Jacobian

SpeedCHEM, direct sparse

SpeedCHEM, Krylov

ns

ns

3

Perini, 2014

27 CEFRC2-3, 2014

3-D CFD: Improved solver numerics


Group thermodynamically-similar cells to reduce the calling frequency to save

computer time - Adaptive Mechanism Clustering (AMC) scheme

Extended dynamic adaptive chemistry (EDAC) scheme

Dynamically determine the size of fuel chemical mechanism based on the

local and instantaneous thermal conditions of the cells

Shi, 2012

Perini, 2014 Efficient chemistry solvers – cell clustering

Chemkin Solver

Remap back

to cells

Thermodynamically

similar cells

(similar temperature,

equivalence ratio )

Liang, 2009

28 CEFRC2-3, 2014


-50 -40 -30 -20 -10 0 10 20 30 40 500.0

0.5

1.0

1.5

2.0

2.5

3.0

Pre

ssure

(M

Pa)

Crank Angle

Experiment

Simulation-Full Chemistry

Simulation-AMC + EDAC model

ERC PRF mech.

(39 sp, 141 rxn)

Full AMC AMC+EDAC

48.27 hrs. 3.99 hrs. 2.88 hrs.

29

HCCI engine validation

Shi, 2012

29 CEFRC2-3, 2014


Spark Ignition Engine

Turbulent Flame Propagation

• G-equation description of combustion

• Laminar and turbulent flame speeds

• Primary heat release calculation

• Flame quench due to mixture stratification

Post-flame Chemistry

• CO oxidation, H2-O2 reactions

• Pollutant formation mechanisms

Knocking Combustion

• Auto-ignition mechanisms

• Location / intensity

SI engine combustion modeling

Flame propagation Models with Detailed Chemistry

Liang, 2006

30 CEFRC2-3, 2014

Spark plug


What is a turbulent flame?

Law, 2014

ST

Ensemble of thin (laminar) flamelets,

interacting with the flow turbulence.

Due to increased surface area, turbulent

flame “brush” propagates at enhanced velocity

L

T L

T

AS S

A

AT

LA

31 CEFRC2-3, 2014


, local ST0 = 0

Quenched broken

reaction zones

Kl l

K Fl l

3 30.1K m m Fl C l C l

Combustion regime diagram

If

Flamelets

Turbulent flame structure

1/ 43

3/ 4ReK Il l

e

0

/ |p T

F

u L

cl

S

Kolmogorov/Batchelor

length scales:

Laminar flame thickness:

Ghandhi, 2012

~ 20 mm

It is not possible to resolve a turbulent flame on a practical engine simulation grid

Peters, 2000

Liang, 2007

32 CEFRC2-3, 2014

Engine flames


Laminar flame speed: balance between reaction and diffusion

iT T

dYS = D

dt

33 CEFRC2-3, 2014

2

2/i i

i

Y YD

t x

20

,21

/N

f i i p

i

T TD h c

t x

( )1/( ) [1/(1 )]

mSx St

mDU x St e

2

2( )

U UD F U

t x

1( ) (1 )m mF U U Ub

1

DS

m

b

Consider the single component system:

unburned

burned unburned

T TU

T T

with and

Admits a traveling wave solution

where

2

( 1)D D m

mS m

b

and

U

x

S

1.0

0

- Mallard, Le Chatelier

~

Part 3: Chemical Kinetics, HCCI & SI Combustion Reitz, 1981

34 CEFRC2-3, 2014

Laminar flame speed

0.90

0.85

0.80

0.75

0.70

0.65

0.60

0.55

Pe

ak B

urn

ing

Ve

locity (

m/s

)

Paraffins Olefins Aromatics

toluene

o,p xylene

Oxygenates

cyclohexane

benzene

anisole

ethanol

MTBE

methylcyclopentane

1,2,4-trimethylbenzene

iso-octene

n-butane

propane

ethane

methane

n-pentane

iso-octane

iso-pentane

iso-butane

n-hexane

t-butylbenzene

i-propylbenzene

m-xylene

ethylbenzene

n-propylbenzene

1-butene

2-pentene

1-hexene

2-butene

3-heptene

cyclopentene

2-methyl-2-butene2-methyl-1-butene

propene

1,3,5-trimethylbenzene

1-pentene

cyclopentane

n-heptane

1-heptene

1-octene

neopentane

C-C-C C=C-C C-C-OH methanol

Part 3: Chemical Kinetics, HCCI & SI Combustion Farrell, 2005

Importance of chemistry - Methane

Flux analysis: oxidation proceeds through methyl

– slow path, high activation energy due to tight C-H3 bonds

Acetylene

Rich

conditions

35 CEFRC2-3, 2014

Part 3: Chemical Kinetics, HCCI & SI Combustion Lutz, 1988

• Limited flux through slow methyl channels

– hydrogen abstraction leaves weaker secondary C-H2 bonds

• Greater flux through chain branching pathways, e.g.,

• Ignition delays are very sensitive to rates of H atom production

H + O2 OH + O

Ignition delay Ethane < Ignition delay Methane

Olefin Acetylene

36 CEFRC2-3, 2014

Importance of chemistry - Ethane

Part 3: Chemical Kinetics, HCCI & SI Combustion Lutz, 1988

Diffusion: Turbulence models

ijji SuuP (RANS - RNG k-e

Wang, 2012

t

ui ui’ + Ui = ui

t l = Ui t k = 3ui

2/2

t k/e

~ turbulent/mean flow time scale

Production

Mean flow strain rate

Reynolds stresses

37 CEFRC2-3, 2014

2 /TD C km e


End Gas

Auto-ignition

(detailed kinetics)

G-Equation

Flame

propagation Burnt Gas

Discrete particle

ignition model

ST from flame speed correlations

GkDGSGvvt

GTT

uvertexf

~~~~)(

~0

Ignition and level set (G-equation) models

Burned gas: G>0

Unburned gas: G<0

Partially Premixed Flame (DI Engine)

Liang, 2007

38 CEFRC2-3, 2014


Φ ≈ 1

Φ > 1

Φ < 1

CO2, H2O, CO, NO…

O2, O, NO …

Fuel Droplets

Diffusion

Diffusion

End-gas Flame Post-flame

Zone Front Zone

CH4, CO, H, H2 …

Turbulent flame speed correlations

1/ 2 1/ 2

22 2

2 24 3 4 34 3

0 1 1

1 1 exp2 2

m ignT

L F F l F

C t t a b a bS l l u la b

S I b l b l S lt

Progress Term

1.5

1 1 0.16 k m I m

kr C l C

e

et

k

K

u

K

F

L

F

r

l

S

u

l

lI

2

151

2/32/1

0

Transition criterion:

Characteristic Timescale:

Stretch factor:

Turbulence stretch Curvature

Discrete Particle

Ignition Kernel

(DPIK) model

rk

Fan, 2000 GkDGSGvv

t

GTT

uvertexf

~~~~)(

~0

Peters, 2000

39 CEFRC2-3, 2014

Part 3: Chemical Kinetics, HCCI & SI Combustion Liang, 2007

Laminar flame speed correlations

Power Law (Metghalchi & Keck, 1982):

dilLL Yp

p

T

TSS 1.21

00

0,

b

)1(8.018.2

)1(22.016.0 b

Liang et al. :

Reference State: 300K, 1bar

220, )( mmL BBS

0.0 0.5 1.0 1.5 2.0 2.50

5

10

15

20

25

30

Equivalence Ratio,

S0

L,r

ef

(cm

/se

c)

Metghalchi et al.

Present study

For iso-octane,

2

,0 exp ( )LS

26.9 -0.134 3.86 1.146

Liang et al.

Liang, 2007

40 CEFRC2-3, 2014


41 CEFRC2-3, 2014

Validation - PFI/DI gasoline engines

Bore × Stroke 89 mm × 79.5 mm

Compression Ratio 12 : 1

Engine Speed 1500 rev/min

PFI Mode

Spark timings (ATDC) -44, -40, -36, -32

MAP (kPa) 65

DI Mode (Spark timing sweeps)

Spark timings (ATDC) -32, -28, -24, -20

MAP (kPa) 75

End of Injection (ATDC) - 72

DI Mode (Manifold-Absolute-Pressure sweeps)

MAP (kPa) 75, 80, 90, 100

Spark timing (ATDC) - 33

End of Injection (ATDC) - 68

DI Mode (End-Of-Injection sweeps)

End of Injection (ATDC) -76, -72, -68, -64

MAP (kPa) 75

Spark timing (ATDC) - 32

Based on MIT PRF Mechanism

(25 species, 51 reactions)

Model constants: Cm1=2.0, Cm2=1.0

(Fixed in all cases)

DI Configuration


Validation - PFI engine operation

Spark Timing = 40 BTDC

Evolution of the G=0 surface

CA = -20 ATDC CA = -5 ATDC

CA = 10 ATDC CA = 20 ATDC

Evolution of Temperature

42 CEFRC2-3, 2014


Validation - PFI engine operation

Spark Timing

-44, -40, -36,

-32 ATDC

Engine Speed

1500 rev/min

-100 -50 0 50 100

0.0

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 50 100

0.0

0.5

1.0

1.5

2.0

2.5

PFI mode

-36 OATDC

Pre

ssu

re (

MP

a)

Crank Angle (oATDC)

EXPT

SIMU

-100 -50 0 50 100

0.0

0.5

1.0

1.5

2.0

2.5

PFI mode

-40 OATDC

Pre

ssu

re (

MP

a)

Crank Angle (oATDC)

EXPT

SIMU

-100 -50 0 50 100

0.0

0.5

1.0

1.5

2.0

2.5

Pre

ssu

re (

MP

a)

Crank Angle (oATDC)

EXPT

SIMU

PFI mode

-44 OATDC

43 CEFRC2-3, 2014


Role of flame propagation

Explore Kinetics-Controlled Formulation for Turbulent Flame Propagation:

After ignition kernel stage, each cell is modeled as a WSR, detailed chemistry is

applied. “Flame propagation” is controlled by heat conduction and auto-ignition.

-80 -60 -40 -20 0 20 40 60 80

0.0

0.5

1.0

1.5

2.0

2.5

Transition from kernel to G-eqn

-20 ATDC

Pre

ssure

(M

Pa)

Crank Angle (oATDC)

EXPT

G-equation

Kinetics only

Spark timing

-44 ATDC

PFI mode

-80 -60 -40 -20 0 20 40 60 80

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

Transition from kernel

to G-eqn at -20 ATDC

DI mode

Spark timing

-32 ATDC

Pre

ssure

(M

Pa)

Crank Angle (oATDC)

EXPT

G-equation

Kinetics only

Mallard-Le Chatelier propagating wave speed: iT

dYS D

dt

44 CEFRC2-3, 2014


Kinetics Controlled G-equation

Summary:

Auto-ignition chemistry alone is

NOT sufficient to properly model

flame propagation.

Turbulence enhancing effect on

flame propagation speed in SI

engines CANNOT be neglected.

PFI case

Spark timing = -44 ATDC

Role of flame propagation

45 CEFRC2-3, 2014


Documents

Reciprocating Internal Combustion Engines Lecture... · prior written permission of the owner, Rolf D. Reitz. Part 3: Chemical Kinetics, HCCI & SI Combustion . Short course outline: