Low Temperature Autoignition of C8

H16

O2 Ethyl and Methyl Esters in a Motored Engine

Yu Zhang and André

BoehmanThe EMS Energy Institute

Penn State University

Motivation

Need for Fundamental Understanding of Unconventional Fuels

2006 BRN Workshop on Basic Research Needs for Clean and Efficient Combustion of 21st Century Transportation Fuels

identified a single over-arching grand challenge:

“the development of a validated, predictive, multi-scale, combustion

modeling capability to optimize the design and operation of evolving fuels in advanced engines for transportation applications.”

Second Generation and Optimal Biodiesel

Howell (DEER 2007) pointed to optimizing the fatty acid ester distribution around mono-unsaturated esters

Knothe

(Energy & Fuels, 2008) recommended genetic modification of the fatty acid profile to fuel property issues simultaneously. Methyl palmitoleate

and esters of decanoic

acid are strong candidates for improving fuel properties besides methyl oleate

Kinney and Clemente (Fuel Processing Technology, 2005) demonstrated that biotechnology can be used to shape the genetics of soybeans to optimize soybean oil for engine performance

Background

Lack of fundamental understanding of the combustion chemistry of

fatty acid esters

Few studies have been focused on the low temperature chemistry of fatty acid esters, which is relevant to application of biodiesel in modern engine designs that employ low temperature combustion strategies

Detailed chemical mechanism of long chain fatty acid esters (e.g. methyl

decanoate) may pose considerable computational challenges for kinetic modeling studies in engine applications

Need to find model compounds for biodiesel that not only exhibit

evident low temperature oxidation behavior, but also have chemical kinetic mechanisms whose sizes are manageable for computational purposes.

Kinetic mechanisms published for methyl butanoate (various authors) and methyl decanoate (Herbinet, Pitz, Westbrook, Combust. Flame,

2008)

Comparisons of ignition characteristics of methyl decanoate with

Fischer-

Tropsch diesel and conventional diesel fuels (DEER 2006 and in Szybist,

Boehman, Haworth, Koga, Combust. Flame,

2007)

Examination of C9 fatty acid esters (Zhang, Yang and Boehman, Combust. Flame, 2009)

Objectives

Investigate the premixed ignition behavior of fatty acid

methyl and ethyl esters in a engine environment

Examine the applicability of the two esters as surrogates for biodiesel in studies of low temperature oxidation

Achieve a fundamental understanding of the major low

temperature oxidation pathways of fatty acid ethyl esters

An important step in the low temperature oxidation of paraffins is the isomerization of RO2

into QOOH through internal H-atom shift by

forming transition state ring-like structures. Six-

and seven-membered rings have the most rapid reaction rate

Experimental Setup

Test Fuels Engine Setup

GDI Injector

Main AirHeater

Heating Tape

Air FlowMeter

VariableCompression

Ratio CFREngine

Exhaust

Up to 260˚

C

Varied Φ

Compression Ratio=4.0-13.75

Combustion Analysis

Optional intake dilution with N2

Test Conditions

GC TCD/FIDAnalysis

GC/MS Analysis

Pin

: 1 atm Tin

: 155 °C

Equivalence Ratio : 0.25

Ignition Progression of Methyl Heptanoate

-0.001

0

0.001

0.002

0.003

0.004

0.005

0.006

0.007

300

400

500

600

700

800

900

300 320 340 360 380 400 420App

aren

t Hea

t Rel

ease

Rat

e (k

J/D

eg) B

ulk Cylinder G

as Temperature (K

)

CA (Deg)

LTHR

-0.001

0

0.001

0.002

0.003

0.004

0.005

0.006

0.007

300

400

500

600

700

800

900

300 320 340 360 380 400 420App

aren

t Hea

t Rel

ease

Rat

e (k

J/D

eg) B

ulk Cylinder G

as Temperature (K

)

CA (Deg)

LTHR

-0.001

0

0.001

0.002

0.003

0.004

0.005

0.006

0.007

300

400

500

600

700

800

900

1000

300 320 340 360 380 400 420App

aren

t Hea

t Rel

ease

Rat

e (k

J/D

eg) B

ulk Cylinder G

as Temperature (K

)

CA (Deg)

LTHR

-0.01

0.01

0.03

0.05

0.07

0.09

0.11

50

250

450

650

850

1050

1250

1450

1650

300 320 340 360 380 400 420App

aren

t Hea

t Rel

ease

Rat

e (k

J/D

eg) B

ulk Cylinder G

as Temperature (K

)

CA (Deg)

LTHR

HTHR

CR=7.05

CR=8.06 CR=9.95

CR=7.69

As CR increases, methyl heptanoate experiences the transition from single

stage LTHR to two-stage ignition

Ignition Progression of Ethyl Hexanoate

Both of the two esters exhibit evident low temperature heat release

-0.001

0

0.001

0.002

0.003

0.004

0.005

0.006

0.007

300

400

500

600

700

800

900

300 320 340 360 380 400 420App

aren

t Hea

t Rel

ease

Rat

e (k

J/D

eg) B

ulk Cylinder G

as Temperature (K

)

CA (Deg)

LTHR

-0.001

0

0.001

0.002

0.003

0.004

0.005

0.006

0.007

300

400

500

600

700

800

900

300 320 340 360 380 400 420App

aren

t Hea

t Rel

ease

Rat

e (k

J/D

eg) B

ulk Cylinder G

as Temperature (K

)

CA (Deg)

-0.001

0

0.001

0.002

0.003

0.004

0.005

0.006

0.007

300

400

500

600

700

800

900

1000

300 320 340 360 380 400 420App

aren

t Hea

t Rel

ease

Rat

e (k

J/D

eg) B

ulk Cylinder G

as Temperature (K

)

CA (Deg)

-0.01

0.01

0.03

0.05

0.07

0.09

0.11

50

250

450

650

850

1050

1250

1450

1650

300 320 340 360 380 400 420App

aren

t Hea

t Rel

ease

Rat

e (k

J/D

eg) B

ulk Cylinder G

as Temperature (K

)

CA (Deg)

LTHR

HTHR

CR=7.05

CR=8.47 CR=9.95

CR=7.69

Ignition Behavior of Test Fuels at CR=7.05

-0.002

0

0.002

0.004

0.006

0.008

0.01

320 330 340 350 360 370 380App

aren

t Hea

t Rel

ease

Rat

e (k

J/D

eg)

CA (Deg)

N-Heptane

Methyl Heptanoate Ethyl

Hexanoate

The presence of ester functionality suppresses low temperature reactivity

Methyl heptanoate is more reactive than ethyl hexanoate in the low temperature

regime due to the higher possible number of 1,5-H and 1,6-H atom transfers available for the isomerization of peroxy alkyl ester radicals during methyl heptanoate oxidation than during ethyl hexanoate oxidation

Gaseous Species Concentrations

Methyl Heptanoate

Ethyl Hexanoate

In the early stage of oxidation, CO is mainly formed through the decomposition of

aldehydes; small olefins are produced through C-C bond

β-scission reactions

The higher concentration of CO for methyl heptanoate in the early stage of oxidation further confirms the higher low-temperature reactivity of methyl heptanoate

More fuel carbon is observed to be converted to ethylene during ethyl hexanoate

oxidation than during methyl heptanoate oxidation despite that ethyl hexanoate is less

reactive than methyl

heptanoate in the low

temperature regime

0

0.5

1

1.5

2

2.5

3

3.5

6.5 7 7.5 8 8.5 9 9.5 10 10.5

CO

and

CO

2 Mol

e Fr

actio

ns (%

)

Compression Ratio

CO2

CO

0

100

200

300

400

500

600

700

6.5 7 7.5 8 8.5 9 9.5 10 10.5

C2H

4 and

C3H

6 Mol

e Fr

actio

ns (p

pm)

Compression Ratio

C2H

4

C3H

6

0

0.5

1

1.5

2

2.5

3

3.5

6.5 7 7.5 8 8.5 9 9.5 10

CO

and

CO

2 Mol

e Fr

actio

ns (%

)

Compression Ratio

CO2

CO

0

100

200

300

400

500

600

700

6.5 7 7.5 8 8.5 9 9.5 10C

2H4 a

nd C

3H6 M

ole

Frac

tions

(ppm

)

Compression Ratio

C3H

6

C2H

4

Condensed Species Collected from the Oxidation of Ethyl Hexanoate at CR= 7.69

Methyl Esters AldehydesEthyl 2-Propenoate Ethyl 3-Butenoate Ethyl 4-Pentenoate Ethyl 3-HexenoateEthyl HexanoateEthyl 2-HexenoateEthyl 2,5-Epoxyhexanoate

Ethyl 3,5-Epoxyhexanoate

Ethyl 2,3-Epoxyhexanoate

2-methyl-5-butyl-1,3-dioxolan-4-one

Acetaldehyde AcroleinPropanalButanal 2-Butenal Pentanal

AcidsFormic AcidAcetic AcidAcrylic AcidHexanoic Acid

Ketone2-Butanone

Cyclic EtherEthylene Oxide

AlkenesPropene1-Butene1,3-Butadiene1-Pentene2-Pentene

Reaction Pathways Analysis

Type Ia. C-C bond scission to form aldehydes, unsaturated esters with a terminal double bond and hydroxyl radicals

Type Ib. C-O bond scission to form unsaturated esters and hydroperoxy radicals

Type Ic. O-O bond scission to form epoxy esters and hydroxyl radicals

1,4-H atom transfer

1,6-H atom transfer

1,5-H atom transfer

1,6-H atom transfer

Type II. reaction pathways leading to low temperature chain branching

The observation of more fuel carbon being converted to C2

H4

during ethyl hexanoate oxidation together with the identification of hexanoic acid among the reaction intermediates from the oxidation of ethyl hexanaoate provide the

evidence for the existence of the six-centered unimolecular elimination reaction during low

temperature oxidation of ethyl esters

Based on the above analysis, major low temperature oxidation pathways of fatty acid ethyl esters can be summarized as follows

Relative Abundance of Unsaturated Ethyl Esters

0

5

10

15

20

25

6.5 7 7.5 8 8.5

Ethyl 2-PropenoateEthyl 3-ButenoateEthyl 4-PentenoateEthyl 3-HexenoateEthyl 2-Hexenoate

Compression Ratio

Mol

e Fr

actio

ns o

f Uns

atur

ated

Est

ers

(% o

f unr

eact

ed fu

el)

The appreciably higher abundance of ethyl 2-propenoate than the rest of the unsaturated esters suggests the abstraction of H-atoms on the α-

carbon of the ester carbonyl

group plays an important role in

the oxidation of ethyl hexanoate

The preferred H-atom abstraction

on the α-

carbon of the ester carbonyl group among the C-H

bond breakings is primarily due to the electron-withdrawing nature of the oxygen atom in the ester carbonyl group

Ethyl 2-propenoate can be formed mainly through the reaction

pathways shown on the left

Ignition Studies of Unsaturated C9 Fatty Acid Methyl Esters

CR=6.08, Tin

= 250 °C, Equivalence Ratio=0.25

Low temperature reactivity follows the

order: Methyl Nonanoate > Methyl 2-

Nonenoate >Methyl 3-Nonenoate

The presence of a double bond in the

aliphatic chain of fatty acid esters inhibits low temperature reactivity; the inhibition effects becomes more pronounced as the double bond moves toward the center of the aliphatic chain

Unsaturated fatty esters are less reactive in the low temperature region due to the fact that the presence of double bonds inhibits the formation of six-

and seven-

membered transition state rings for

internal H-atom transfers that are

important in low temperature oxidation

-0.002

0

0.002

0.004

0.006

0.008

0.01

320 330 340 350 360 370 380 390 400App

aren

t Hea

t Rel

ease

Rat

e (k

J/D

eg)

CA (Deg)

-0.001

-0.0005

0

0.0005

0.001

0.0015

330 340 350 360 370 380 390 400App

aren

t Hea

t rel

ease

Rat

e (k

J/D

eg)

CA (Deg)

-0.001

-0.0005

0

0.0005

0.001

0.0015

330 340 350 360 370 380 390 400

App

aren

t Hea

t Rel

ease

Rat

e (k

J/D

eg)

CA (Deg)

Methyl Nonanoate

Methyl 3-Nonenoate

Methyl 2-Nonenoate

Selected Condensed Species Collected from the Oxidation of Methyl 3-Nonenoate at CR=5.54

The presence and high abundance of hexanal and methyl 2-propenoate implies methyl 3-nonenoate experiences the following olefin-like reaction sequence

Species Relative Peak AreaMethyl 2-Propenoate 0.16Butanal 0.3Pentanal 0.2Hexanal 1

LTHR magnitude is dependent on CN and LTHR magnitude is dependent on CN and equivalence ratioequivalence ratio

0

5

10

15

0.0 0.5 1.0 1.5 2.0

LTH

R (%

of t

otal

)

Equivalence ratio

○

#2 diesel-light cut◊

FT diesel-light cut■

methyl decanoate

••

LTHR magnitude trends with derived cetane numberLTHR magnitude trends with derived cetane number

FT diesel-light cut > methyl decanoate > #2 diesel-light cut

••

Methyl decanoate LTHR likely overMethyl decanoate LTHR likely over--predicts LTHR of biodieselpredicts LTHR of biodiesel–

Aliphatic chain is responsible for LTHR, not methyl ester–

Over 50% of soy-based biodiesel is comprised of species with multiple unsaturations

–

Unsaturated species exhibit less LTHR than saturated species

CH3

O

O

CH3

methyl decanoate

Szybist and Boehman, DEER 2006

2

4

6

8

10

12

14

5.5 6 6.5 7 7.5 8 8.5

LTH

R%

Compression Ratio

Ethyl Nonanoate

Methyl Nonanoate

Methyl 2-Nonenoate

Conclusions

The alkyl chain of fatty acid esters experience the typical paraffin-

like low temperature oxidation sequence; the alkyl chain length of fatty acid esters has a crucial impact on the ignition behavior of fatty acid esters

The six-centered unimolecular

elimination reaction occurs during the early stage oxidation of fatty acid ethyl esters

The abstraction of H-atoms on the α-carbon of ester carbonyl

group plays an important role in the oxidation of fatty acid esters

The presence of a double bond in the aliphatic chain of fatty acid esters suppresses the low temperature reactivity of fatty acid

esters; the inhibition effect becomes more pronounced as the

double bond moves toward the center of the aliphatic chain of fatty acid esters

For unsaturated esters, the autoignition

can undergo olefin ignition pathways

Acknowledgements

National Biodiesel Board for support of this study

Infineum

USA L.P. for support of the C9 methyl ester study

Dr. Yi Yang (now at Sandia National Laboratory) and Dr. Dania Fonseca of Penn State University

Prof. Rodolphe

Minetti

of Université

des Science et Technologies de Lille