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Nanoparticle Emissions from Internal Combustion Engines
Professor David B. KittelsonDepartment of Mechanical Engineering
University of MinnesotaMPLS, MN 55455 USA
presented at
The Royal Society Discussion MeetingUltra Fine Particles in the Atmosphere
15 March 2000 Engine Exhaust Particle Emissions:
Some Current Issues
Outline
• Background
• Formation and character of exhaust particles
• Nuclei mode formation and growth
• Role of carbonaceous agglomerates• Summary and Conclusions
Most of the recent U of M experimental work reported here was performed by Dr. Imad Abdul-Khalek and Mr. Qiang Wei with funding from Perkins Engine Company
Emissions of Ultrafine and Nanoparticles from Engines• 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
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
AccumulationMode
CoarseMode
PM10Dp < 10 µm
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 1.0000
Particle Diameter ( µµm)
dN
/d(l
og
(Dp
)) (
par
t./c
m3 )
1991 engine, low S fuel 1988 engine low S fuel 1979 Roadway study
The new engine increased number emissions 10 to 30 fold!
HOWEVER, 1979 roadway measurments made on behind a truck powered by anengine of the same family showed highnanoparticle emissions! Other roadsideand on road measurements made since the late 60's have shown high nanoparticle emissions
The new engine sharply reduced mass emissions, by about a factor of 3.
Normalized Particle Size Distributions from 1999 Tuscarora Tunnel Study, Courtesy Alan Gertler, Desert Research Institute
1
10
100
1000
1 10 100 1000
Diameter (nm)
d N
/d lo
gD
13% HD
65% HD
78%HD
Mainly Light-DutySpark Ignition
Mainly Heavy-Duty Diesel
Nanoparticles in the atmosphere appear to be associated with both spark ignition and diesel engines.
Outline
• Background
• Formation and character of exhaust particles
• Nuclei mode formation and growth
• Role of carbonaceous agglomerates• Summary and Conclusions
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
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
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
Particle Formation History: From the Start of Combustion to the Nose
University of Minnesota
This is where most of the nanoparticles emitted by engines
usually form.
Outline
• Background
• Formation and character of exhaust particles
• Nuclei mode formation and growth
• Role of carbonaceous agglomerates• Summary and Conclusions
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
Influence of Residence Time onNumber Weighted Size Distributions
1.00E+05
1.00E+06
1.00E+07
1.00E+08
1.00E+09
1.00E+10
1 10 100 1000
Dp (nm)
DN
/DL
og
Dp
(P
art.
/cm
³)
Residence time = 1000 ms 100 ms 230 ms
Tdilution = 32 °C, Primary DR ~ 12
1600 rpm, 50% load
Influence of Dilution Temperature on Number Weighted Size Distributions
1.00E+05
1.00E+06
1.00E+07
1.00E+08
1.00E+09
1.00E+10
1 10 100 1000
Dp (nm)
DN
/DL
og
Dp
(P
art.
/cm
³)
Tdil (32 °C) Tdil (48 °C) Tdil (65 °C)
Residence Time ~ 400 ms, Primary DR ~ 12Humidity Ratio ~ 0.0016
1600 rpm, 50% load
Influence of Primary Dilution Ratio on Number Weighted Size Distributions
1.00E+05
1.00E+06
1.00E+07
1.00E+08
1.00E+09
1.00E+10
1 10 100 1000
Dp (nm)
DN
/DL
og
Dp
(P
art.
/cm
³)
Primary Dilution Ratio (PDR) ~12
PDR ~ 25
PDR ~ 40
Primary Dilution Temperature (PDT) ~ 48 °C,
Residence time ~1000 ms, HR ≤≤ 0.0016
1600 rpm, 50% 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)
Which Volatile Materials in the Exhaust Are Likely Precursors to Nuclei Mode Formation and Growth?
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
For typical diesel exhaust conditions, saturation ratio of hydrocarbons comprising the SOF may not be high enough for nucleation, S > ~ 3
• 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
tica
l Co
nce
ntr
atio
n R
atio
C/C
crit
Tex = 150 C
240 C
330 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%
Influence of Fuel and Lube Oil Sulfur (courtesy C.Barnes, Perkins Engine Company)
1.00E+05
1.00E+06
1.00E+07
1.00E+08
1.00E+09
1.00E+10
1 10 100 1000
Dp (nm)
dN
/dlo
g(D
p)
par
t/cm
3) 1
07 Jan 2000, 450 ppm Sulphur, Normal Lube Oil
11 Nov 1999, 3 ppm Sulphur, Normal Lube Oil
16/18 Nov 1999, 3 ppm Sulfur Fuel + Low Sulphur Lube Oil
Medium-Duty Engine, 1600 rpm, 10% Load
Sulfuric Acid Appears To Trigger Nucleation But Typical Concentrations Are Too Low To Produce Observed Particle Growth Rates
0
100
200
300
400
500
600
700
0 0.2 0.4 0.6 0.8 1
Time (s)
Vap
or
Den
sity
( µµg
/m3)
0
5
10
15
20
25
30
35
40
45
Dro
ple
t D
iam
eter
(n
m)
Acid vapor density Hydrocarbon vapor densityDp acid/water Dp hydrocarbon
Maximum rates of particle growth and vapor depletion in free molecularregime. 400 PPM sulfur fuel, 3% sulfur to sulfate conversion, 15% RH, 0.4 m2/m3 soot surface area, 8 mg/m3 C19H40 as hydrocarbon, Dilution ratio 12, T dilution 32 C
Growth rates of about 20 nm/s are observed for these fuel and dilution conditions
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.
Outline
• Background
• Formation and character of exhaust particles
• Nuclei mode formation and growth
• Role of carbonaceous agglomerates
• Summary and Conclusions
Carbonaceous Agglomerates Adsorb Volatile Particle Precursors and Suppress Nanoparticle Nucleation and Growth
• A time constant for molecular adsorption may be determined from the molecular collision frequency and the particle surface area available for adsorption.
• Under typical roadway conditions the dilution ratio, D, of an exhaust plume varies roughly linearly in time so that a time constant for dilution can be defined as follows.
• Then a dimensionless adsorption rate can be defined.
= pa A
MRT
5.0
2/1
πτ
dtD τ/1+=
adadsR ττ /=
H2SO4 Nucleation and Growth Is Suppressed by Adsorption onto Carbon Agglomerates
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
Low exhaust carbon emissions and/or fast (roadway) dilution.More potential to form nanoparticles.
High exhaust carbon emission and/orslow (laboratory, congested traffic) dilution.Less potential to form nanoparticles.
H2SO4 / H2O nucleation
500 PPM S fuel, 3% conversion
Texh =330 C, Tair = 27 C, RH =40%
Outline
• Background
• Formation and character of exhaust particles
• Nuclei mode formation and growth
• Role of carbonaceous agglomerates• Summary and Conclusions
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 particle precursors that are in the vapor phase inthe 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. Carbon agglomerates provide a large surface area for adsorption that suppresses nucleation.
– Thus, engines with low carbon mass emissions may have high number emissions of volatile, but NOT solid particles.
– It should be possible to reduce nanoparticle emissions by reducing emissions of sulfuric acid and hydrocarbons.
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. Even larger changes may occur with engines that emit very little carbon.
– Carbon in the exhaust suppresses nanoparticle formation less effectively when the exhaust is diluted rapidly over a roadway than when it is diluted slowly.
• 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(l
og
(Dp
)) (
par
t./c
m3 )
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