Unique Properties of Gasoline Direct Injection (GDI)

Engine Particulate Matter (PM): Morphology,

Nanostructures, and Oxidative Reactivity

April 30, 2014

Heeje Seong*, Kyeong Lee and Seungmok Choi

Center for Transportation Research

Argonne National Laboratory

2014 Cross-Cut Lean Exhuast Emissions Reduction Simulations (CLEERS) Workshop

Fritz, AVL Seminar

[Khalek et al. SAE 2010-01-2117]

Objectives

Examine GDI PM properties at various

enigne conditions, which are

unanalyzable by typical measurement

instruments.

Evaluate effects of physicochemical properties on GDI particulate oxidation.

Background

[Stewart at PNNL, 2011 AMR]

2

Approach

TEM analysis

Thermophoretic sampling (Residence time: 20 ms min.)

GDI engines

Soot oxidation experiments

with TGA

TGA

DSC

Raman analysis

Synchrotron X-ray at APS (PDF analysis)

FTIR-ATR analysis 3

GDI engine generates particulates in a wide size range

1500 rpm & 50% load (15,000x)

190° bTDC 300° bTDC 330° bTDC

Numerous sub-23 nm particles are found from a stock GDI engine.

(quantitative data are given in the next slides)

4

Diesel particulates

Seong and Boehman (2011, Energy & Fuels)

Influences of engine operating conditions (speed/

load, inj. time) are apparent on particle sizes

Primary and aggregate particles tend to grow in

size with advancing injection timing at the fixed

spark timing.

There emit a high population of sub 23 nm

particles from GDI engines.

0

10

20

30

40

50

60

10 20 30 40 50 60

190 (o bTDC)

230 (o bTDC)

260 (o bTDC)

300 (o bTDC)

330 (o bTDC)C

ou

nt

(%)

dp (nm)

50

10

20

30

40

50

20 50 100 200 300 400

190 (o bTDC)

300 (o bTDC)

330 (o bTDC)

Co

un

t (%

)

Rg (nm)

10

Radius of gyration:

𝑅𝑔 =1

𝑛 𝑟𝑖

2𝑛𝑖=1 and 𝑛 =

𝐴𝑎

𝐴𝑝

1.09

𝑛: # of primary particles per aggregate

0

5

10

15

20

25

30

35

40

0

20

40

60

80

100

120

140

160

180 200 220 240 260 280 300 320 340

dp

Rg

dp

Rg

Injection timing (o bTDC)

2100 rpm 1500 rpm/50% load (spark timing fixed)

5

0

10

20

30

40

50

60

10 20 30 40 50 60

Aggregates with small dp

Gasoline, = 0.98

Gasoline, = 1.13

E20, = 0.98

E20, = 1.13

Co

un

t (%

)

Rg (nm)

5

65% load 310° bTDC

HR-TEM analysis proves that a number of sub-23 nm

particles are well-defined solid carbons

15,000X 500,000X Processed Image

Background: amorphous

carbon film

600,000X

Ca : from lubricant oil

12,000X

Cu & Au : from grid support

Ca

6

Ca

Au

Cu

Au

GDI particles are much smaller than conventional

diesel particles, comparable to LTC particles

0

50

100

150

200

0 200 400 600 800

GasolineLD DieselHD Diesel

Ag

gre

gate

part

icle

siz

e,

Rg (

nm

)

Exhaust Temperature (oC)

10

20

30

40

50

60

0 200 400 600 800

GasolineLD dieselHD diesel

Pri

ma

ry p

art

icle

siz

e,

dp (

nm

)

Exhaust temperature, Texh

(oC)

LTC

LTC GDI

GDI large particles

GDI nanoparticles

(1st gen GDI)

7

iB16

E85

E10

Gasoline

Alcohol-derived soot particles are quite different

from gasoline-derived soot in nano-structure

Diesel

E10

2000 RPM, 75% load

8

GDI particulates undergo continuous expansion

under electron beams of high-magnification TEM

Particulate expansion was

observed more often with

alcohol-derived soot.

Organic compounds dissolved

in the particles developed into

amorphous soot.

This behavior should affect

oxidative reactivity of GDI soot.

10.5 nm 16.5 nm

3000 RPM, 50% load

After 600,000X

Gasoline-derived soot Alcohol-derived soot

9

1500 rpm-25% load

1500 rpm-50% load

1500 rpm-75% load 3000 rpm-75% load

Diesel soot (50% load)

GDI soot indicates clear concentric carbon fringe patterns, regardless of

engine operating condition.

Gasoline-derived GDI soot appears to be graphitic

similar to diesel soot in TEM observation

600kX

Cold start

10

100

120

140

160

180

200

Printex-UGDI: cold idle

GDI: 1250-25%-330o

GDI: 1500-50%-190o

GDI: 1500-50%-330o

Diesel: 50% load

D1 F

WH

M (

cm

-1)

Raman analysis revealed that GDI soot has similar

internal structures at various engine conditions

0

0.2

0.4

0.6

0.8

1

1000 1500 2000

Printex-U

Diesel: 50% load

GDI: cold idle

GDI: 1250rpm-25%

GDI: 1500rpm-50%

No

rma

lize

d i

nte

ns

ity

Raman shift (cm-1

)

Raman analyses - 633nm; Curve-fitting method - 3 Lorentzian & 1 Gaussian bands

Raman peak patterns of various GDI soot samples are nearly the same in a small

deviation, while those of diesel soot are dependent on engine conditions.

− Different combustion processes between GDI and diesel engines may be responsible

for the difference.

Overall the degree of carbon crystalline structures of GDI soot is quite similar among

various engine operating conditions.

0

0.2

0.4

0.6

0.8

1

800 1000 1200 1400 1600 1800 2000

1800rpm-15%1800rpm-50%1800rpm-75%

No

rmali

zed

In

ten

sit

y

Raman shift (cm-1

)

Increase in degree of disorder

Diesel soot G D

11

PDF analysis was first used to prove the finding from Raman

analysis (similar atomic structures regardless of engine conditions)

Pair distribution function (PDF) analysis by the high energy synchrotron X-ray enables the

accurate evaluation of local- and medium-range structures of various engine soot samples. − Higher peak intensities below ~5Å: more ordered local atomic structure.

− Higher peak intensities over ~5Å: bigger crystalline domains resulting from lateral and stacking coherence.

PDF data verify Raman results: − Degree of crystalline structures: GDI soot< Diesel soot

− GDI soot shows no distinct change with engine conditions.

0 5 10 15 20

Ato

mic

PD

F, G

(r)

Radial distance, r (Å)

Printex-U (PU)

GDI: 1250-25-330

GDI: cold start

GDI: 1500-50-190

GDI: 1500-50-330

0 5 10 15 20

Ato

mic

PD

F,

G(r

)

Radial distance, r (Å)

Graphite

Carbon black A

LTC soot

GDI soot: 50% load

diesel soot: 50% load

A B

C

Pyrocarbon model Weisbecker et al.

(Carbon, 2012, p.1563)

A B C

12

0.008

0.01

0.012

0.014

0.016

0.018

0.02

0.022

0 0.2 0.4 0.6 0.8 1 1.2 1.4

1/t

80 (

min

-1)

Ash content (%)

1500/50%-190o

3000/50%-330o

1500/75%-330o

1500/50%-330o 1500/25%-330

o3000/50%-190

o

diesel soot: 2000/50%

Oxidation reactivity of GDI soot is closely related to

ash fraction, which decreases with soot concentration

TGA experiments confirm that GDI soot reactivity increases with ash content.

In spite of more ordered structure, diesel soot appears to be more reactive than GDI soot

at a similar ash content will be discussed in the next slide.

The soot oxidation effect of ash content will be significant at normal hot-steady engine

conditions, because of high ash fractions relative to low soot concentrations.

0

20

40

60

80

100

120

0 0.5 1 1.5 2 2.5 3 3.5 4

So

ot

co

nc

en

trati

on

(m

g/m

3)

Ash content (%)

3000/50-330o

1500/25-330o

1500/50-330o

3000/50-190o

1500/75-330o

1250/25-330o

1500/50-190o

1250/25-default

speed/load-inj. timing

0

20

40

60

80

100

0 50 100 150 200 250

Re

ma

inin

g m

as

s (

%)

Time (min)

GDI: 1500/50%-190o

3000/50%-190o

1500/25%-330o

1500/50%-330o

1500/75%-330o

3000/50%-330o

<Oxidation with 8% O2 at 600

oC>

Diesel: 2000/50%

t80

13

The role of surface chemistry on soot oxidation

could be limited in the presence of ash

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

800100012001400160018002000

Ab

so

rban

ce

(A

.U.)

Wavenumber (cm-1

)

GDI: 1500/50%-330o

GDI: 1500/50%-190o

Diesel: 2000/50%

GDI: 3000/50%-330o

Carbonyl C=OAromatic C=C

Aliphatic C-H

C-O-C from ether

C-O from ester and alcohol

Overall the GDI and diesel soot samples turned out to contain a comparable amount of

surface functional groups.

Further investigation is needed to confirm that diesel soot is more reactive than GDI soot.

14

Müller et al. (Catalysis Today, 2012)

Proposed soot oxidation process

Conclusions

15

TEM examinations were performed for GDI particulates from various

operating conditions.

− GDI particulates were distributed in wide size ranges.

− A number of sub-23 nm particles emitted from GDI engines were proven to

be solid carbon.

GDI particulates appeared to contain a high amount of organic

compounds, which promote further soot growth under appropriate

temperature conditions in supply of external energy.

− GDI particulates from certain conditions only experience particle expansion

(e.g. alcohol blends-derived soot and certain gasoline-derived soot).

Raman and PDF analyses confirmed that crystalline structures of GDI

soot are in a same level, regardless of engine operating condition.

− Fuel effects to be further examined.

Oxidation reactivity of GDI soot was enhanced with ash content.

− Ash content increased with reduced soot concentration.

− Ash effects on soot oxidation will be significant at normal hot-steady

engine operations due to high ash fractions.

Acknowledgements

Funding and Support

− U.S. DOE Office of Vehicle Technologies, Gurpreet Singh

and Ken Howden

− Corning Inc. and Hyundai motor company

− IEA-Advanced Motor Fuels Program

Instrument use of the Center for Nanoscale Materials and

Advanced Photon Sources was supported by the U. S. DOE

Office of Science, Office of Basic Energy Sciences, under

Contract No. DE-AC02-06CH11357.

16