Measurement of Engine Exhaust Particle Size

David B. Kittelson

Center for Diesel Research

University of Minnesota

presented at

University of California, Davis

17 February 2000

Background

• Current emission standards are mass based. Recently interest in other measures, i.e, size, number, surface, has increased.

• Concerns about particle size– New ambient standards on fine particles– Special concerns about ultrafine and nanoparticles– Indications that reductions in mass emissions may increase number

emissions

• Difficulties associated with measurement of ultrafine and nanoparticles– Often more than 90% of particle number are formed during exhaust

dilution– Particle dynamics during sampling and dilution are highly nonlinear -

large changes in of particle number may result from small changes dilution and sampling conditions

0

0.02

0.04

0.06

0.08

0.1

0.12

0.14

0.16

0.18

0.2

0.001 0.010 0.100 1.000 10.000

Diameter (µµm)

No

rmal

ized

Co

nce

ntr

atio

n, d

C/C

tota

l/dlo

gD

p

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1

Alv

eola

r D

epo

sitio

n F

ract

ion

Mass Weighting Number Weighting Alveolar Deposition Fraction

Fine Particles

µUltrafine ParticlesDp < 100 nm

NanoparticlesDp < 50 nm

NucleiMode

AccumulationMode

CoarseMode

PM10Dp < 10 µm

Fractional deposition of particle with density of 1 g/um

Typical Diesel Particle Size Distribution

Typical Diesel Particle Size Distribution - Log Scale

Typical Engine Exhaust Size DistributionBoth Mass and Number Weightings are Shown

0.00001

0.0001

0.001

0.01

0.1

1

0.001 0.010 0.100 1.000 10.000

Diameter (µµ m)

No

rmal

ized

Co

nce

ntr

atio

n, d

C/C

tota

l/dlo

gD

p

Mass Weighting Number Weighting

Fine ParticlesDp < 2.5 µm

Ultrafine ParticlesDp < 100 nm

NanoparticlesDp < 50 nm

NucleiMode Accumulation

Mode

CoarseMode

PM10Dp < 10 µm

Health

• Correlations between fine particles and excess deaths

• Increased asthma in children living near roadways

• Special concerns about ultrafine and nanoparticles

Special Concerns about Nanoparticles and Health

• Increased deep lung deposition

• Increased number and surface area at same mass exposure

• Particles which are non-toxic in µm size range may be toxic in nm range– Rats exposed to the same mass of 0.25 and 0.02µm diameter ΤiΟ2

retain more nanoparticles in the interstitial tissue of the lung and develop marked inflammatory response. (Seaton et al., 1995).

– Comparison of surface free radical activity of ambient PM10 particles, and 0.5 and 0.025 µm diameter TiO2 showed significant activity for PM10 and much more activity for 0.025 than for 0.5µm TiO2 (Donaldson et al., 1996).

– Modest concentrations of 0.03µm Teflon fume particles caused acute pulmonary toxicity in rats (Ferin et al., 1992).

A Recent HEI Study Gave a Surprising Result for a Low Emission Engine

• Particle concentrations and size distributions were measured for a 1988 and a 1991 engine– When run on very low sulfur fuel particle number emissions were much

higher, 30 -100 times, from the new engine although mass emissions were about 3 times lower

– The 1988 engine produced high number emissions, but not as high as the 1991 engine when run on a 1988 type fuel (higher sulfur)

– This raised concerns that new engines might be producing large numbers of nanoparticles while still meeting mass emission standards and focused attention on nanoparticle emissions

• However reviews of measurements made on and near roadways in the 70’s and 80’s show high nanoparticle emissions. High nanoparticle emissions may be a problem but are not a new development!

Number Size Distribution Data from HEI Report and 1979 CRC Roadway Study

1.0E+06

1.0E+07

1.0E+08

1.0E+09

1.0E+10

0.0010 0.0100 0.1000 1.0000Particle Diameter (µµm)

dN

/d(lo

g(D

p))

(p

art./

cm3 )

Old engine, low S fuel

1979 Roadway measurments

New engine, low S fuel

Particles consist mainly of highly agglomerated solid carbonaceousmaterial and ash and volatile organic and sulfur compounds.

Hydrocarbon / Sulfate Particles

Sulfuric Acid Particles

Solid Carbonaceous / Ash Particles withAdsorbed Hydrocarbon / Sulfate Layer

0.3 µm

Particle Composition and Structure

Typical Composition of Diesel Particulate Matter: Lube oil contributes to SOF, ash, sulfate

60%10%

10%

13%7%

Carbon

Ash

Sulfate and Water

Lube Oil SOF

Fuel SOF

Particle Formation History: From the Start of Combustion to the Nose

Carbon formation/oxidationt = 2 ms, p = 150 atm.,

T = 2500 K

Ash Condensationt = 10 ms, p = 20 atm.,

T = 1500 K

Exit Tailpipet = 0.5 s, p = 1 atm.,

T = 600 KSulfate/SOF

Nucleation and Growtht = 0.6 s, p = 1 atm.,D = 10, T = 330 K

Fresh Aerosol overRoadway–Inhalation/Aging

t = 2 s, p = 1 atm.,D = 1000 T = 300 K

IncreasingTime

Formation

Atmospheric AgingExposure

University of Minnesota

Significant Gas to Particle Conversion Takes Place as the Exhaust Dilutes and Cools

• More than 90% of the particle number may form through homogeneous nucleation of nanoparticles

• From 5 to more than 50% of the particle mass may form through adsorption, absorption, and nucleation

• These processes are extremely sensitive to dilution conditions

• Thermodynamics and fluid mechanics of mixing during dilution are very complicated and vary widely

A dilution ratio of 1000 may be reached in 1 - 2 s

Atmospheric Dilution Leads to Nucleation, Absorption, and Adsorption

Studies of Diesel Nanoparticle Formation Using a Variable Residence Time Dilution System

Engine

CPC

PrimaryEjectorPump

Insulated Variable Residence Time

section

Diluted NOx

SMPS

OverflowBypass

Three Way Valve

SecondaryOrifice

SecondaryEjector Pump

OverflowBypass

PressureGauge

PrimaryOrifice

ExhaustPipe

Tygon Flexible

Tube

Air InletThermocouple

Humidity and temperature

Sensor

PressureGauge

Emission Rack, CO2,

NOx

Three Way Valve

SamplingProbe

Thermo-couple

Retractable tubing

Heated or Cooled Compressed Air Compressed

Air

Sensitivity of Diesel Particle Size Distribution to Dilution Conditions - Residence Time Effects

1.00E+06

1.00E+07

1.00E+08

1.00E+09

1 10 100 1000

Dp (nm)

DN

/DL

og

Dp

(P

art.

/cm

³)

900 ms

400 ms

90 ms

Heavy-Duty Diesel

Medium Load and Speed

Primary dilution ratio 12

Temperature 48 °C

Sensitivity of Diesel Particle Number Emissions to Dilution Conditions - Residence Time and Temperature Effects

3350

65

90

400

900

0.00E+00

1.00E+08

2.00E+08

3.00E+08

4.00E+08

5.00E+08

6.00E+08

7.00E+08

Number Concentrations

(Part./cm³)

Dilution Temperature (°C)

Residence Time (ms)

Heavy-Duty DieselMedium Load and Speed

Primary Dilution Ratio = 12

Nanoparticle formation downstream of an exhaust filter may be even more sensitive to dilution conditions

1.00E+03

1.00E+04

1.00E+05

1.00E+06

1.00E+07

1.00E+08

1.00E+09

1 10 100 1000

Dp (nm)

DN

/DL

og

Dp

(P

art.

/cm

³)

Upstream, Res. 40 ms Upstream, Res. 6000 ms

Dow nstream, Res. 40 ms Dow nstream, Res. 6000 ms

Mode 6

Upstream

Downstream

Figure 11

Peak torque

speed, full load

Our results suggest nanoparticles are mainly volatile, we have demonstrated this with a catalytic stripper

Typically more than 99% of the nanoparticles disappear by 300 C (except with metal additives)

1.000E+06

1.000E+07

1.000E+08

1.000E+09

1.000E+10

1 10 100 1000

Dp (nm)

DN

/DL

og

Dp

(Par

t./c

m³)

Engine Out-1000 ms Engine Out-40 ms

Stripper-160 °C-1000 ms Stripper-295 °C-1000 ms

Engine Out (Res. 1000 ms)

Stripper - 295 °C

Stripper– 160 °C

Engine Out (Res. 1000 ms)

Engine Out (40 ms)

The saturation ratio, S, is defined as:

vpp

S =

where p is the partial pressure and pv is the vapor pressure of the volatile component. The dilution ratio,D, is defined as:

exh

mix

Q

QD =

where Qmix and Qexh refer to the standard volumetric flows of diluted and raw exhaust, respectively.Also:

Dp

1∝

and

mixRT

H

v ep∆−

∝so that

mixRTH

De

S ∆−∝1

and for adiabatic dilution.

How do nanoparticles form during dilution? Driving force for gas-to-particle conversion is saturation ratio

For typical diesel exhaust conditions, saturation ratio may not be high enough for nucleation, S > ~ 3

Influence of Dilution Upon Extractables

0

0.05

0.1

0.15

0.2

0.25

0.3

0.35

0.4

0.45

1 10 100

Dilution Ratio

Sat

ura

tio

n r

atio

Sat. Ratio Adiab. Sat. Ratio

Assumes extractable mass of 8 mg/m3 is nonodecaneTex = 240 C, Ta = 27 C

These levels are too low for nucleation but would drive absorption and adsorption

• An extreme dependence of the nucleation rate upon temperature, H2SO4, and H2O concentrations makes theoretical predictions of nucleation difficult.

• Seinfeld and Pandis give an empirical expression to predict the onset of nucleation in this system:

Ccrit = 0.16exp(0.1T - 3.5RH - 27.7)where Ccrit is the threshold H2SO4 concentration in µg/m3, T is temperature, RH is relative humidity (0 to 1).

• I have used this expression and mass and energy balances applied to the dilution process to predict the ratio of the actual H2SO4 concentration to the critical one, C/Ccrit. When this critical concentration ratio exceeds one, nucleation is likely.

What does nucleate? Consider the threshold for Binary H2SO4-H2O nucleation

Influence of Fuel and Exhaust Conditions on Sulfuric Acid Nucleation

0.01

0.1

1

10

100

1 10 100 1000 10000

Dilution ratio

Cri

tical

Con

cent

ratio

n R

atio

C/C

crit

Tex = 150 C

240 C330 C

Tex = 150 C

240 C

330 C

Nucleation likely for C/Ccrit > 1

500 ppm Fuel Sulfur

5 ppm Fuel Sulfur

Fuel sulfur conversion to sulfate = 3%,

Air-fuel ratio = 30, Tair = 27 C, RH = 40%

H2SO4 Nucleation Is Suppressed by Adsorption on Carbon Particles

1.00E-02

1.00E-01

1.00E+00

1.00E+01

1.00E+02

1 10 100 1000 10000

Dilution Ratio

Co

nce

ntr

atio

n R

atio

C/C

crit

Rads = 0 0.01 0.02 0.08 0.27 0.92 3.20

Nucleation suppressed by increasing carbon concentration or slower dilution

Increasing dimensionless adsorption rate

H2SO4 / H2O nucleation

500 ppm S fuel, 3% conversion

Texh =330 C, Tair = 27 C, RH =40%

Nanoparticle Nucleation and Growth

• It appears that with current engines binary sulfuric acid - water nucleation triggers the process.

• The initial size of these nuclei is about 1 nm.

• In most cases there is not enough sulfuric acid present in the exhaust to explain the observed rates of particle growth.

• Hydrocarbons normally associated with the soluble organic fraction apparently are absorbed by the concentrated sulfuric acid nuclei leading to the observed growth rates.

Summary Diesel - 1

• A significant amount of particulate matter (e.g. 90 % of the number and 30% of the mass) is formed during exhaust dilution from material present in the vapor phase in the tailpipe (e.g., sulfuric acid, fuel and oil residues). – New particles are formed by nucleation. This is likely to be the source

of most of the ultrafine and nanoparticles (and particle number)associated with engine exhaust.

– Preexisting particles grow by adsorption or condensation.

– Nucleation and adsorption are competing processes. Soot agglomerates provide a large surface area for adsorption that suppresses nucleation. Thus, engines with low soot mass emissions may have high number emissions.

Summary Diesel - 2

• Nanoparticle measurements are very strongly influenced by the sampling and dilution techniques employed.– Nucleation, adsorption, absorption, and coagulation during sampling

and dilution depend upon many variables, including dilution rate, (or residence time at intermediate dilution ratio), humidity, temperature, and relative concentrations of carbon and volatile matter.

– Changes of more than two orders of magnitude in nanoparticle concentration may occur as dilution conditions are varied over the range that might be expected for normal ambient dilution, e.g., 0.1 to 2 s dilution time scales.

• Sampling systems should mimic atmospheric dilution to obtain samples representative of the tailpipe to nose process.

The most difficult problem associated with exhaust particle measurement is understanding the dilution process from tailpipe to nose -- this is being examined in the CRC E-43 program

This illustration shows a typical application of the University of Minnesota Mobile Aerosol laboratory developed for the CRC E-43 Program

Nanoparticles are produced by new engines, but concentrations may be much lower than feared

1.0E+02

1.0E+03

1.0E+04

1.0E+05

1.0E+06

1 10 100 1000

Particle Diameter (nm)

dN

/d(lo

g(D

p))

(p

art./

cm3 )

Cruise

Hard Acceleration

Summary Diesel - 3

• Currently most of the particles in the nanoparticle size range are volatile. However, as engines become cleaner, metallic ash particles from the lubricating oil (or fuel if metallic additives are present) may become more important.– Solid particles raise new issues…

– But they are easy to control with exhaust filters

• Spark ignition engines typically emit smaller particles than diesel engines and are an important source of fine particles andnanoparticles.– A recent study in Colorado concluded that up to 2/3 of the fine particle

mass emitted by vehicles was from spark ignition engines– New gasoline direct injection engines emit much higher particle

concentrations than conventional engines and may approach diesellevels under some conditions

Particles from Spark Ignition Engines - Approximate Composition of Exhaust Particulate Matter

80%

10%5% 5%

Unresolved complexmixture (UCM)*Ash

Sulfates, carbon, etc.

Oxygenated and PAC

* Includes branched and cyclic compounds

Well Maintained Port Fuel Injection Engines

Nanoparticle Emissions from Spark Ignition Engines Also Depend upon Dilution Conditions

1.0E+04

1.0E+05

1.0E+06

1.0E+07

1.0E+08

1.0E+09

1 10 100 1000

Particle Diameter, D p [nm]

Nu

mb

er C

on

cen

trat

ion

, dN

/dlo

g(D

p)

[par

ticl

es/c

m3 ]

DR = 10.9DR = 11.7DR = 15.3DR = 20.5DR = 25.4

DR = Dilution Ratio

Port Fuel Injection, 4 Valve per Cylinder

Port Fuel Injected Spark Ignition Engine -Influence of Load and Additives

Figure 2. Brake Specific Particle Number Emissions as a Function ofPower with 95% Confidence Intervals

0.0E+00

1.0E+13

2.0E+13

3.0E+13

4.0E+13

5.0E+13

6.0E+13

7.0E+13

0 5 10 15 20 25 30 35 40 45Power [kW]

Bra

ke S

pec

ific

Par

ticle

Nu

mb

er E

mis

sio

ns

[par

t./kW

-hr]

non-additized

OX13391C

OX13003L

95% Confidence Interval: +/- ~10%

Port Fuel Injected Spark Ignition Engine - Influence of Additives on Size Distributions

Figure 3. Representative Baseline Size Distributions for the OX13391 andOX13003 Additives [2500 RPM, 90 kPa]

0.0E+00

5.0E+05

1.0E+06

1.5E+06

2.0E+06

2.5E+06

3.0E+06

3.5E+06

4.0E+06

4.5E+06

5.0E+06

0.001 0.01 0.1 1Particle Diameter, dp [µµ m]

Exh

aust

Num

ber C

once

ntra

tion,

dN

/dlo

g(d

p)

[par

ticle

s/cm

3 ]non-additizedOX13391COX13003L

Summary - Particle Emissions from Port Fuel Injection (PFI) Spark Ignition Engines

• PFI engine exhaust particles are quite different from diesel particles– They usually smaller

– They are composed primarily of volatile materials– Lube oil may play an important role

• PFI emissions are strongly influenced dilution conditions– Thus they are formed from volatile precursors during dilution

– Sulfuric acid-water nucleation and hydrocarbon absorption likely play a role although direct nucleation of heavy hydrocarbon derivatives may play a role

– Formation likely to be associated by local inhomogeneous conditions -big droplets, crevices

0.0

0.2

0.4

0.6

0.8

1.0

0 10 20 30 40 50Brake Power [kW]

Pen

etra

tio

n T

hro

ug

h C

atal

ytic

C

on

vert

er

Particle Number Penetration through the Catalytic Converter on a Port Fuel injection Engine

Variable Speed and Load

Time-Resolved Polydisperse Number Concentration in an SI engine Under Steady State Operating Conditions

0.0E+00

2.0E+06

4.0E+06

6.0E+06

8.0E+06

1.0E+07

1.2E+07

1.4E+07

1.6E+07

1.8E+07

2.0E+07

0 500 1000 1500 2000Time [seconds]

Exh

aust

Par

ticl

e C

on

cen

trat

ion

[p

artic

les/

cm3 ]

Navg = 1.11 x 106 particles/cm3

2.3L PFI SI Engine2000 RPM, 55 kPa MAP

~ 5 x 105 part/cm3

Stable and Unstable (Spiking) Particle Size Distributions from a Spark Ignition Engine

1.00E+03

1.00E+04

1.00E+05

1.00E+06

1.00E+07

1.00E+08

1.00E+09

0.001 0.01 0.1 1

Particle Diameter [µµm]

Exh

aust

Par

ticl

e N

um

ber

Co

nce

ntr

atio

n,

dN

/d(l

og

dp)

[p

arti

cles

/cm

3 ]

Baseline Aerosol Spiking Aerosol

DGNSPIKE = 11.4 nm

DGNBASELINE = 35.3 nm

Size Distributions for a GDI Engine

1.00E+05

1.00E+06

1.00E+07

1.00E+08

1.00E+09

10 100 1000Particle Diameter, D p [nm]

dN

/dlo

g(D

p) [

par

ticl

es/c

m3 ]

13 km/hr 32 km/hr 50 km/hr 70 km/hr 90 km/hr

Summary - Particle Emissions from GDI Spark Ignition Engines

• These engines are likely to come into widespread use in Europe and Asia

• Particles structure and emission rates are much more like those from diesels than PFI engines

• Only limited studies of fuel and additive effects have been done