April 30, 2014 1

Detailed Characterization of Particulates

Emitted by Lean-Burn Gasoline Direct

Injection Engine

Alla Zelenyuk, Jacqueline Wilson, Mark Stewart, George Muntean

Pacific Northwest National Laboratory

John Storey, Vitaly Prikhodko, Samuel Lewis, Mary Eibl

Oak Ridge National Laboratory

Relevance and Objectives

Limited fossil fuel resources and upcoming U.S. fuel

economy and emission standards are major challenges

in current engine development

Small displacement turbocharged Gasoline Direct

Injection (GDI) engines are replacing large displacement

engines, particularly in light-duty trucks and sport utility

vehicles, with future lean-burn GDI engines potentially

offering even higher fuel economy than stoichiometric

GDI engines

Unfortunately, all GDI engines produce higher particulate

emissions than standard port fuel-injection engines

There is lack of actual emissions characterization data

on pre-commercial and future combustion engines

We present advanced aerosol analysis methods applied

to the characterization of particulates emitted by a 2.0L

lean-burn GDI engine operated using a number of

combustion strategies (lean homogeneous, lean

stratified, stoichiometric, and fuel rich conditions)

GDI engine

miniSPLAT

Sampling Setup

2008 BMW 1-series 120i, 87 AKI

Gasoline, 2.0L, naturally aspirated

Three-Way Catalyst (TWC)

Fuel EEE Cert (low S) 97 RON, 89 MON

FTIR on raw exhaust

Single stage dilution for filters

DR 1st stage: 12-15; 2nd stage: 140-160

Engine Points (PM measured with and w/o TWC):

• 2000 rpm @ 2, 4, & 8 bar;

1000 rpm @ 1 & 2.5 bar

• lean stratified, lean homogeneous,

stoichiometric, and rich

Engine

To Vent

FTIR

Micro-Tunnel Diluter

Two Stage Diluter (PMP)

Stage 1 – 150° C

Evaporator – 400° C

TWC

Raw Pre-Catalyst

Raw Post-Catalyst

Diluted

Zefluor

Filters

SMPS SPLAT II TEM

1

2 Stage 2 - 50°C

Advanced Characterization Setup

Real-time, in-situ, detailed particulate matter (PM) characterization:

SMPS:

• size distributions (mobility

diameters, dm )

SPLAT II:

• single particle size (vacuum

aerodynamic diameter, dva)

• single particle composition, MS

DMA/SPLAT II:

• effective density, ρeff

• fractal dimension, Dfa

• primary spherule diameter, dp

APM/DMA/SPLAT II:

• particle mass, mp

• fractal dimensions, Dfm, Dpr

• primary spherule diameter, dp

• number of spherules, Np

• void fraction, Φ

• shape (χt , χv ) Zelenyuk et al. (2014) Combustion and Flame, doi:10.1016/j.combustflame.2014.01.011.

Characterizes particles with sizes from

50 nm to 3 µm (50% cut-off at 85 nm)

Sizes up to 5,000 particles/sec

Sizes particles with 0.5% precision

Measures the composition of 20-100

particles/sec

Uses IR/UV ion formation mode to yield

quantitative particle composition,

material density, morphology

Characterizes mobility and/or mass

selected particles to yield information on

particle effective density, mass, fractal

dimension, dynamic shape factors,

average primary spherule diameter,

number of spherules, void fraction

SPLAT II

Single-Particle Mass-Spectrometer

Zelenyuk et al. (2014) Combustion and Flame, doi:10.1016/j.combustflame.2014.01.011

Zelenyuk, A., Yang, J., Imre, D. and Choi, E. (2009). Aerosol Science and Technology, 43:5,411-424.

Zelenyuk, A., and Imre (2009). International Reviews in Physical Chemistry, 28(2):309-358.

Lean Stratified GDI PM

Point 1: Lean stratified, 2000 rpm, 2 bar, Engine out

Mobility size-distribution

Total concentration:

5.4e+07 particles cm-3

Vacuum aerodynamic size distribution of all particles produced during lean stratified

engine out run peaks at 60 nm

It is significantly narrower than mobility size distribution, indicating that the majority

of the particles are fractal

vacuum aerodynamic diameter nm vacuum aerodynamic & mobility diameters nm

Lean Stratified GDI PM

Point 1: Lean stratified, 2000 rpm, 2 bar, Engine out

Particles are composed of elemental carbon (>80%), small amount of organics

(very little HC, mostly carboxylic acids, little PAHs), and some Ca from lube

detergent additives

All exhaust particles are internally mixed, but there are clear classes

PM Compositions

Point 1: Lean stratified, 2000 rpm, 2 bar, Engine out

Not all exhaust particles are the same: different compositions and size distributions

Compositions of Individual

Exhaust Particles

Composition-resolved

Size Distributions

When particle mobility diameters increase from 80 nm to 250 nm (312%), their

vacuum aerodynamic diameters increase from 59 nm to only 75 nm (27%)

Effective density is a function of mobility diameter, with slope that yields fractal

dimension (Dfa = 2.18)

This relationship can also be used to determine the average primary spherule

diameter of soot agglomerates, which in this case yields, dp = 22 nm

vacuum aerodynamic diameter nm mobility diameter nm

Aerodynamic diameters of

Mobility-Selected Particles Point 1: Lean stratified, 2000 rpm, 2 bar, Engine out

When particles masses increase from 0.3 fg to 3 fg, their vacuum aerodynamic

sizes increase from 53 nm to only 74 nm

This relationship can also be used to determine the average primary spherule

diameter of soot agglomerates (Shapiro et al. 2012, Zelenyuk et al. 2014)

vacuum aerodynamic diameter nm va

cuu

m a

ero

dyn

am

ic d

iam

eter

nm

Aerodynamic diameters of

Mass-Selected Particles Point 1: Lean stratified, 2000 rpm, 2 bar, Engine out

The power relationship between particle mass and mobility diameter yield another

measure of fractal dimension and particle effective density

These particles have Dfa = 2.18, and Dfm = 2.12 (Dfa and Dfm of spherical particles

is 3, of infinite linear chain agglomerates is 1, and of ideal fractal particles is 2)

mobility diameter nm mobility diameter nm

Point 1: Lean stratified, 2000 rpm, 2 bar, Engine out

Mobility Diameters of Mass-

Selected Particles

Particle Void Fraction and

Number of Spherules

The number of spherules in these soot agglomerates are a power function of the

mobility diameter

Larger particles have lower effective density, and more voids, with void fraction

reaching 80%

mobility diameter nm mobility diameter nm

Nu

mb

er

of

sp

heru

les

Vo

id f

racti

on

Point 1: Lean stratified, 2000 rpm, 2 bar, Engine out

2000 rpm, 2 bar

Lean Stratified and Stoichiometric Lean stratified

Engine out

Stoich

Engine out TWC

TWC

PM before and after TWC have very similar properties

Stoichiometric PM contains higher fraction of larger Ca-dominated particles

Dfa 2.18

Dfm 2.12

Dfa 2.17

Dfm 2.08

Dfa 2.22

Dfm 2.10

vacuum aerodynamic diameter nm

vacuum aerodynamic diameter nm

2000 rpm, 2 bar

Lean Stratified and Stoichiometric Lean stratified Stoichiometric

Stoichiometric: Lower number concentrations, presence of volatile particles

Stoichiometric: Catalyst removes nuclei mode

Engine out, total concentration:

5.4e+07 particles cm-3

mobility diameter nm mobility diameter nm

Engine out, total concentration:

4.6e+06 particles cm-3

2000 rpm, Stoichiometric

Different loads

dva increases

The fraction of EC particles increases

Engine out

2 bar

TWC

4 bar

8 bar

mobility diameter nm

Dfa 2.22

vacuum aerodynamic diameter nm

At higher loads:

At the highest load dm decreases

The total number of particles decreases

1000 rpm, 1 bar

Lean Stratified and Stoichiometric

Stoichiometric PM contains higher fraction of larger Ca-dominated particles

TWC slightly decreases dva of soot particles

Lean stratified

Engine out

Stoichiometric

Engine out TWC

TWC vacuum aerodynamic diameter nm

vacuum aerodynamic diameter nm

Ca/C

aO

2

2000 rpm, 4 bar

Lean Homogeneous and Stoichiometric Lean homogeneous

Engine out

Stoichiometric

Engine out TWC

TWC

Lean homogeneous PM contains higher fraction of larger Ca-dominated particles and

of organic particles

vacuum aerodynamic diameter nm

Ca/C

aO

2

Rich Operation

2000 rpm, 4 bar

dva distributions are nearly independent of l

At higher l PM number concentrations are significantly higher

At 0.9 l the fraction of EC particles is slightly higher

0.98 l

Engine out

0.9 l

Engine out TWC

TWC

mobility diameter nm

vacuum aerodynamic diameter nm

l

Preliminary Conclusions

The study yielded high quality, detailed data on the properties of particles emitted by the 2.0L BMW lean-burn GDI engine operated under more than 20 conditions

In addition to the measurements of PM number concentrations and size distributions, we characterized the vacuum aerodynamic size, mass, compositions, and effective density of individual exhaust particles

These measurements were used to calculate fractal dimensions, average diameters of primary spherules that comprise soot agglomerates, and the number of spherules, void fraction, and dynamic shape factors as function of particle size

Most of the particles produced by the GDI engine are fractal containing small amount of oxygenated organics and PAHs

The fraction of Ca-containing particles, originating from the detergent additives to lubricating oil, varies with operating conditions and has a very distinct size distribution

Explored differences between PM emitted by GDI in three operating modes: lean stratified, lean homogeneous, stoichiometric

•Lean stratified operation yielded the most diesel-like size distributions

•Stoichiometric operation resulted in PM number concentrations an order of magnitude lower than those emitted under lean stratified operation

•Stoichiometric PM contains a higher fraction of Ca-dominated non-fractal particles

•Lean homogeneous PM contains a high fraction of Ca-dominated and organic particles