measurement of engine exhaust particle size
TRANSCRIPT
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