notes on „soot“ measurement of diesel engines · 2017. 7. 18. · soot measurement of diesel...
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Notes on „Soot“Measurement of Diesel Engines
Wolfgang SchindlerWolfgang Singer
AVL List GmbH, Graz, Austria
Soot measurement of Diesel engines 2004.-08-17 | Page 2
Why bother about Soot Measurement ?
The measurement of “Soot” is a widely accepted method in Engine R&D since several decades. It provides information about the quality of the combustion processPMP has decided that agglomeration mode particles should be counted - Such particles typically consist of “soot” in Diesel exhaustThere is some evidence that soot is more relevant with regard to health concerns than the homogeneous, volatile “Nanoparticles”
HC Fuelcond.
Soot “ C “
Metal & Others
WaterHC Oilcond.
Gravimetric measurement oftotal particulate mass
after partial-flow or full-flow dilution
Soot measurement of Diesel engines 2004.-08-17 | Page 3
Properties of Soot from Diesel Engines
Soot is a combustion product. (“Carbon black” is no “soot”). Soot consists of “Carbonaceus Particles”Soot has a combustion temperature in air of >450 °C,it is not volatile or liquid.(compare the measuring proposal of PMP)The size is mainly in the “agglomeration mode” range, the structure irregular (fractal dimension 1.8 to 2.5) Soot is “really black”
Soot measurement of Diesel engines 2004.-08-17 | Page 4
Methods for Diesel Soot Measurement
Analysis of filtered Diesel Particulates
– chem. extraction– thermogravimetry– Coulometry
PMP method:Thermodesorption and particle counting
Via its optical properties:– The mass extinction coefficient
of Soot is nearly constant for D < 100 nm
– The scattering coefficient goes with D/λ - 4
Mass extinction with Lamda= 560 nm
1.E-02
1.E-01
1.E+00
1.E+01
10 100 1000Diameter (nm)
Mas
s ex
t. co
eff R
(m2/
g)
ScatterExtinktion
typical mean size of Diesel particulate
Soot measurement of Diesel engines 2004.-08-17 | Page 5
How well can various systems measure Soot Mass? C
arbo
nC
arbo
n „C
“ „
C“
„HC“„HC“
SOSO44--
Ash
F ilte
r Gra
vim
e try
F SN
Op a
city
PhotoacousticSensor
Opaci-meter
Smoke Meter
Criteria:accuracy - selectivity -measuring artefactsconcentration:
– resolution, detection limit– dynamic range
repeatability, reproducibilitytime resolution
Target: to assess the physical limits of various methods and the practical problems encountered when measuring Diesel Soot.
Soot measurement of Diesel engines 2004.-08-17 | Page 6
“Filter Smoke Number (FSN)” Measurement
Advantages:– Simple method, proven reliability,
widely used in R&D– Selectivity: for soot content >15%, FSN is
clearly related to Soot concentration– Detection limit: with high sampling volume
20 µg/m3 or 0,002 FSN achievable⇒ dynamic range > 1: 10000 (concentration)
– Repeatability: 5% achievable (see below)
Challenges:– Due to the simplicity of the method it is
often used uncritically without observing the basic principles of particle measurement.
– A realistic theory of the relation between FSN and Soot concentration does not exist
Drawback:time resolution >10 sec by principle
currentenginesfutüre
FSN
0 1 2 3 4 5
Soot measurement of Diesel engines 2004.-08-17 | Page 7
FSN measurement: Challenges
Influences to FSN Measurement:– Sample Conditioning. FSN is measured from the raw exhaust ⇒
concentrations are high, particle deposition is of considerable concern.– Hangup after measurement of large FSN: purging is required to avoid
re-entrainment during “white level determination” ( <2 µg allowed !)– Filter paper: if two different filter papers, at opposite ends of the ISO
10054 specification band are used, results may vary by >20%.(AVL makes a special paper check to guarantee ± 5%).
– Soot content: in aerosols with a very small soot core (<15% by mass) “the absorption coefficient is increased”
FSN reproducibility ESC, Euro 5 HD engine
0.00
0.02
0.04
0.06
0.08
0.10
0.12
1 2 3 4 5 6 7 8 9 10 11 12 13 mode
FSN
ESC 251
ESC 250
ESC 234
ESC 235
If measurements are carried out carefully, even the emissions of a Euro 5 HD engine can be measured reproducibly.
Soot measurement of Diesel engines 2004.-08-17 | Page 8
Opacity Measurement
Advantages:– Good time resolution, ≤ 0.1 sec– Theoretically well defined ( for fixed R)– Detection limit: k = 0.002 m-1
corresponding to ≈ 300 µg/m3
(sufficient for non-trap engines)Challenges:
– To achieve a stable “Zero” value I0– The mass extinction coefficient R of
soot is slightly size dependent (typical uncertainty ± 20%)
– The influence of NO2 on the signal is substantial,the influence of HC and SO4 only at high concentrations
Drawback:– At the current stage the technology is
at its limits
Soot measurement of Diesel engines 2004.-08-17 | Page 9
Challenges for good Opacity Sensitivity
I/ I0= e -k . L = ( 1 - N/100 )N [%] Opacityk [m-1] Absorption
Good resolution in k is achieved if:• I0 is stable• The measuring length L is large
To some extent the two requirements contradict each other:
hot
hot
cold
If the temperature of gas and chamberwalls are not stable, the thermal lens(„mirage effect“) changes I0Temperature changes of 1.5 °C resultin an I0 change of 0.1%
U(Det) Änderung mit Gas-ein T
y = 0.0547x + 0.5119
-0.5
2.0
4.5
-20 30 80T(Zelle) - T(Gas-ein)
% U
(Det
)
Soot measurement of Diesel engines 2004.-08-17 | Page 10
“Accuracy” - is Opacity related to Soot?
Practical experience:A constant (± 20%) relation betweenOpacity and Soot is given if:• NO2 concentration is < 100ppm• The particulates consist to less than
50% of HC and SO4
• The exhaust temperature in the measuring chamber is ≥ 100°C
• The particle size is in the “typical”Diesel exhaust accumulation mode range
In other words: isk (m-1) = R • Conz (mg/m3)
with konstant R?
FSN_Opazitätskorrelation
y = 0.0043x + 0.0403
0.00.10.20.30.40.50.60.70.80.9
0 50 100 150 200mg/m3 aus FSN
k-W
ert
BMW Werte
Prüfstand 13
Prüfstand 4
MAN Werte
Linear (BMW Werte)
Quantities influencing R:• Particulate size.• Particulate composition: (HC and
SO4)
The Particulate size dependence of R decreases with increasing λ-> IR extinction (drawback: R decrease with increasing wavelength)
Comparison of Partial and Full-flow Opacimeters to Smoke values FSN < 0.1
-0,1
0,0
0,1
0,2
0,3
0,4
0,5
0,6
0,7
0 0,02 0,04 0,06 0,08 0,1 0,12FSN
N (%
)
Partial Flow 430 mmFull-flow, 6 inch
Soot measurement of Diesel engines 2004.-08-17 | Page 11
How about light Scattering?
I0
Is IS = Fkt( ϑ, ϕ, d/λ, n, ff)with n = m+ik,
ff = form faktor
IS ∝ d-6
In Principle: The measurement of Is at different ϑ, ϕ and at different λ canyield a wealth of information about size, size distribution, number, shape.....
In Practice: • Single particle scattering: with reasonable effort the lowersize limit is ≈ 100 nm due to low scattering intensity.
• Multi particle scattering: large particles dominate the Signal.• Multi-wavelength and -angle scattering: the addition of
measuring uncertainties introduces ambiguities into thedata evaluation.
⇒ A robust and accepted light scattering instrument has not made its way to the market despite substantial and repeated effort in the last 20 years
Soot measurement of Diesel engines 2004.-08-17 | Page 12
Photoacoustics - AVL Micro Soot Sensor
Periodic heating by a modulated laser beam and subsequent cooling generates an acoustic signal that is detected by a microphone
Advantages:– Sensor is “directly” sensitive to soot
concentration– high sensitivity
(detection limit < 10 µg/m³)
Modulatedlaser beam
Soot particle
Modulatedheating
Modulatedexpansion
Acoustic waveDetector
microphone
Challanges:– signal evaluation at low
concentrations– repeatability / reproducibility /
calibration– currently for diluted exhaust only
Soot measurement of Diesel engines 2004.-08-17 | Page 13
Accuracy: Selectivity, Linearity and Comparability
Selectivity:No cross-sensitivity from typical or untypical exhaustgases, if the proper near IR Laser wavelength is used (H2O, HC, SO4, NO2....).
Due to the physical principle thesignal is linear with sootconcentration
Linearity:
Comparability:According to experience in various laboratories the signal iscomparable to the sootdetermined with gravimetric sootmass determination
Comparability of Photoacoustics with
Gravimetry
y = 0.9773x - 6E-05R2 = 0.9914
0.00
0.01
0.02
0.03
0.00 0.01 0.02 0.03PM soot mass / g km-1
AVL4
83 /
g km
-1
Courtesey V. Scheer, FFAResearch & Advanced Engineering
Soot measurement of Diesel engines 2004.-08-17 | Page 14
Reproducibility, Repeatability, Calibration
Reproducibility:The near IR frequency of the laser used guarantees, that D/λis small for typical Diesel particulates. Therefore the mass extiction coefficient R is nearly constant
The experiments show that twophotoacoustic systems of different build and different calibrationmethods show equal results
Repeatability:
Calibration:Calibration must be performed in comparison to gravimetric methods. However, the calibration factor in the„PASS“ has not been changed in thelast 2 years, and yields the sameresults as the AVL Micro Soot Sensor
Courtesey V. Scheer, FFA
Comparability of two Photoacoustic instruments
(IWC Munich and AVL)
y = 0.9912x + 0.0004R2 = 0.9963
0.00
0.01
0.02
0.03
0.00 0.01 0.02 0.03PASS / g km-1
AVL
483
/ g k
m-1
Research & Advanced Engineering
Soot measurement of Diesel engines 2004.-08-17 | Page 15
Resolution (concentration, time)
ETC 3 (without PM_KAT) and 5,6 (with PM_KAT)
0
500
1000
1500
2000
1200 1400 1600 1800time(s)
µg/
m3 ETC 3
ETC 5ETC 6
Resolution:Challenges: Good resolution (below10 µg/m3) can be obtained by:– Signal evaluation with high
frequency selectivity– Low noise in the gas path– Correct signal evaluation,
subtracting the baseline signal fromsooted windows phase-corrected
Photoacoustic Signal Evaluation
01
234
567
89
0.0 0.2 0.4 0.6 0.8mg/m3
Sign
al (m
V)
naivphase sensitive
filtered air, but dirty windows
Dynamic range:Photoacoustics allows a dynamicrange of 1: 10000 (important to detect peaks on low background
Time resolution:1 sec τ90 is easily achievable, bettertime resolution currently sacrificesconcentration detectivity
Soot measurement of Diesel engines 2004.-08-17 | Page 16
Laser Induced Incandescence (LII)
ExhaustExhaustpipepipe
ExhaustExhaustadapter ringadapter ring
Laser
PMTPMTDetectorDetector
Gas flowGas flow
High sensitivity for ultra-low emissiongood time resolution for transient testingIn-situ raw exhaust gas measurement
However:– substantial costs, due to expensive
laser– serious laser hazards
Soot measurement of Diesel engines 2004.-08-17 | Page 17
Summary
Several Methods exist to measure Soot on the basis of its strong absorption coefficient.If these methods are used with attention and some basic knowledge about particulate measurement they can give
– reliable soot data– even for very low soot
concentration in the exhaust The method proposed by PMP for measuring particles seems feasible -the interlaboratory exercise has not yet started - but practically it measures only the soot fraction of the Diesel exhaustA second thought on mass-proportional soot measurement methods as a viable alternative seems appropriate.
Mass extinction with Lamda= 560 nm
1.E-02
1.E-01
1.E+00
1.E+01
10 100 1000Diameter (nm)
Mas
s ex
t. co
eff R
(m2/
g)
ScatterExtinktion
typical mean size of Diesel particulate
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