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
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