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Measurement and Characterisation of Aerosols
from Engine Combustion Processes
Dr Jon Symonds Technical Manager
Cambustion, U.K.
AIRBOURNE PARTICULATE MATTER — GENERATION, MEASUREMENT AND IMPACT
29th MAY 2012 — INSTITUTE OF PHYSICS, LONDON
Contents
• Overview of engine particulate emissions
- Characteristics
- Formation
- Evolution
• Legislation – the drive for measurement
• What do we wish to measure & how can we measure it?
- Overview of engine emissions testing
- Mass
- Opacity
- Number
- Size
- Density
• Control technologies
• Future Developments
Types of IC Engines
• Spark Ignition (Gasoline / LPG
/ CNG)
- Port fuel injected:
Stoichiometric air:fuel ratio
required for 3-way catalyst
operation, low P.M. emission
(pre-mixed flame)
- Gasoline Direct Injection
(GDI), higher compression,
greater efficiency…
- … but fuel / air less well
locally mixed → more P.M.
(semi-diffusion, semi-pre-
mixed)
• Compression Ignition (Diesel)
- Spontaneous ignition, due to
compression temperature
- Diffusion flame
- Mixture locally rich in fuel
plume as mixing occurs
• Two-stroke
- Air, fuel and oil mixed
- P.M. high due to burnt
(and aerosolised) oil
Particle emissions from Diesel engines
• Number or mass weighted size spectrum contains 3 modes:
D.B. Kittelson, Centre for Diesel Research, University of Minnesota, U.S.A.
• Engine:
• Carbonaceous
• Organic
• Sulphate
• Ash
• Other (Coarse):
• Brake dust
• Tyre wear
• Road wear
• Rust & scale
Particle formation in engines – soot
• Air:fuel ratio (AFR) critical – “rich” or “lean”
• Stoichiometric HC combustion produces H2O & CO2
• As oxygen levels drop: CO2 → CO → C
• Short timescales: localised AFR can be v. different to overall AFR
based on Dec (2007)
Particle formation in engines – ash, volatile fractions
• Ash formed from inorganic molecules in fuel, oil, additives + engine
wear e.g. metal oxides. Tend not to aggregate, < 50 nm
• Sulphuric acid formed from sulphur in lube oil and fuel, which forms
SO2 during combustion, followed by
𝑆𝑂2 +1
2𝑂2 → 𝑆𝑂3
𝐻2𝑂𝐻2𝑆𝑂4
Catalysed by Diesel Oxidation Catalyst…
• Organic fraction:
- Un-burnt or partially burnt fuel (fuel pools, PAH)
- Lubrication oil (both from cylinder and crankcase, inc. coarse mode
droplets)
• Volatile fractions do no enter the “particulate phase” until
cooling and dilution
Particle evolution – exhaust and beyond
• Cooling: Increase in saturation ratio, drives nucleation and growth
• Dilution: Decrease in partial pressures, suppresses nucleation and
growth
• Initial cooling caused nucleation, “real world” dilution eventually
freezes (or reverses) process
DMS50 Raw Tailpipe / CVS Sampling Data
0.00E+00
2.00E+07
4.00E+07
6.00E+07
8.00E+07
1.00E+08
1.20E+08
1.40E+08
1 10 100 1000
Dp (nm)
Raw (Tailpipe) sampling
Dilution tunnel sampling
dN/dlogDp /cc
Legislation
Stage Date
(for type approval)
Diesel Direct Injection Gasoline
PM (mg/km) PN (#/km) PM (mg/km) PN (#/km)
Euro 3 Jan 2000 50 - - -
Euro 4 Jan 2005 25 - - -
Euro 5a Sept 2009 5* - 5* -
Euro 5b Sept 2011 5* 6.0 × 1011
Euro 6 Sept 2014 5* 6.0 × 1011 5* 6.0 × 1012
(6.0 × 1011
from 2017)
European light duty particulate emission standards – Source dieselnet.com. *4.5 mg/km if measured by the PMP procedure
US:
• Mass based standards for light duty, heavy duty, non-road marine and rail
• California does its own thing – mass based but now considering number
Europe:
Euro V / VI : The Particle Measurement Programme
• United Nations Economic Commission for Europe (UNECE)
commissioned – Introduced a Solid Particle Number metric
• Addresses limitations in and criticism of filter paper method
- Lack of sensitivity & repeatability for ever cleaner engines – mass mostly
condensed gas phase volatiles
- Biased to larger particles, yet smallest particles have greatest health effects
Mass Number Data courtesy of Chris Parkin, UK DfT
The Emissions Lab – Dynamometer Testing
• Dynamometer supplies load to engine
• Vehicle driven on standard driving “cycle”
Federal Test Procedure (FTP) 75
0
10
20
30
40
50
60
70
80
90
100
0 500 1000 1500 2000 2500Time (s)
Speed (
KP
H)
New European Drive Cycle (NEDC)
0
20
40
60
80
100
120
140
0 200 400 600 800 1000 1200
Time (s)
Sp
ee
d (
KP
H)
The Emissions Lab – Sampling and Dilution
• Constant Volume Sampler (CVS) Tunnel:
• Variable dilution of exhaust (variable exhaust flow, constant CVS flow)
• All particles (or gas molecules) end up in CVS flow; CVS flow is known and
constant:
mass (or number) rate = mass (or number) concentration × CVS flow
mg/s = mg/cc × cc/s
total mass = ʃ mass rate dt
specific mass (mg / km) = total mass / distance
Mass Measurement (1)
• Gravimetric (Filter Paper)
• Filter Paper Reflectance
(Smoke Number)
e.g. AVL 415 Weigh before and after
Mass Measurement (2)
• Photo Acoustic Soot Sensor /
Micro Soot Sensor
• Tapered Element Oscillating
Microbalance (TEOM)
e.g. AVL 483 MSS
Filter
Quartz
Tube
Excitation /
Detection
Transducers
e.g. Air Monitors
TEOM-FDMS
Tuned resonant cavity
Opacity
e.g. AVL Opacimeter
• Used for MOT testing
• Still used for some approval regulations
Number Measurement
3790
VPR evaporates liquid particles to less than 23nm
CPC has declining counting efficiency below 40nm (e.g.
only 50% of 23nm particles are counted
Output from CPC corresponds closely to solid particle
number concentration
e.g. Horiba SPCS,
AVL Particle Counter (APC) 489
Condensation Particle Counter (CPC)
Size Measurement
• Aerodynamic Diameter
Electrical Low Pressure Impactor
(ELPI – Dekati):
• Electrical Mobility (drag:charge)
Diameter:
• Scanning Mobility Particle Sizer
(SMPS, e.g. TSI or Grimm)
• Differential Mobility Spectrometer
(fast)….
Differential Mobility Spectrometry (1)
• Particles charged by corona charger
• Particles carried by sheath flow, deflected by high voltage electrode
• Particles land on 1 of 22 detection rings, which measure small currents
• Inversion algorithm converts electrical mobility to size spectrum
• Mass and number calculated from size spectrum
Differential Mobility Spectrometry (2)
Commercial versions: Cambustion DMS500 & DMS50; TSI EEPS & FMPS
West Virginia University
0.0E+00
5.0E+11
1.0E+12
1.5E+12
2.0E+12
2.5E+12
3.0E+12
0 10 20 30 40 50 60 70 80 90 100
Time (s)
Part
icle
Num
ber/
Second
Particle Number from DMS500 Measuring Raw Post-catalyst
Particle Number from PMP Measuring in Dilution Tunnel
The Need for Fast Response
• DMS responds to fast changes in AFR control due to fast time response
Centrifugal Particle Mass Analysers
Commercial versions: Cambustion CPMA & Kanomax APM 3600 and 3601
Diagram courtesy of J. Olfert
eENq
i
oqr
rr
eV
N
m
ln22
2mr
CPMA Density Measurements
50 nm Diesel soot
0.0E+00
5.0E+03
1.0E+04
1.5E+04
2.0E+04
2.5E+04
3.0E+04
3.5E+04
1.00E-02 1.00E-01 1.00E+00
Mp (fg)
dN
/dlo
gM
p/c
c
Effective density
0
100
200
300
400
500
600
700
800
900
1000
0 50 100 150 200 250
Dp (nm)
Eff
ec
tiv
e D
en
sit
y (
kg
/m3)
CPMA
CPC
DMA
Aerosol
Signal
1 lpm
1.5 lpm
Mass:mobility relationship
Mp ∝ Dp2.6
0.0
0.5
1.0
1.5
2.0
2.5
3.0
0 50 100 150 200 250
Dmo (nm)
Mp (
fg)
Symonds et al., 2011
Control Technologies
• Control by engine development / calibration
- EGR
- Injection strategy
- Boost control
• Control by filtration: Diesel Particulate Filter (DPF)
- Good solution for removing solid particles (effectively mandated by
Euro VI for Diesel)
- Require periodic regeneration
S. Payne, 2011
DPF filtration efficiency
DPF Filtration Efficiency - DPG
65
70
75
80
85
90
95
100
0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2
soot load g
Num
ber based filtration efficiency %
Pore bridging mode
Cake filtration mode
Gasoline Direct Injection
dN/dlogdp /cc
1.E+2 2.5E+3 6.3E+4 1.6E+6 4.E+7 1.E+9
1000
500
200
100
50
20
10
5
Part
icle
dia
mete
r
(nm
)
0
20
40
60
80
100
120
140
0 10 20 30 40 50 60 70 80 90 100
Vehic
le S
peed
(kph)
0.0E+00
5.0E+12
1.0E+13
1.5E+13
2.0E+13
2.5E+13
0 200 400 600 800 1000 1200
Time (s)
Part
icle
Num
ber
Particle Number from DMS500
Particle Number from PMP System
Diesel Limit - 6.6x1012
N (6 x 1011
(N/km)
Peckham, Campbell & Finch, 2011
Future Developments (1)
• World harmonised test cycle (WLTP – World harmonised Light
vehicles Test Procedure)
• In use compliance, via Portable Emissions Measurement Systems
(PEMS)
• Europe
- Adoption of WLTP
- Real Driving Emissions (RDE)
- Non-road Mobile Machinery (NRMM)
• USA
- LEV3 (Low Emissions Vehicle) – California
- Tier 3 emissions (Light duty)
- Tier 4 – Non-road
Future Developments (2) – In use compliance
• Real world driving highly transient, and “unrepeatable”
1.00E+06
5.10E+07
1.01E+08
1.51E+08
2.01E+08
2.51E+08
3.01E+08
0 200 400 600 800 1000 1200 1400 1600 1800Time (s)
Co
nc (
n/c
c)
0
10
20
30
40
50
60
70
80
Sp
eed
(m
ph
)
Accumulation mode Concentration Road
Road Speed (MPH)
DMS50 data
Symonds et al. 2007
Conclusions
• Europe moving from mass to number, US may follow
• Fast time response vital for development work using transient cycles
• DPFs effectively solve solid P.M. emissions from Diesel
• Debate continues as to the wisdom of ignoring the small particles in
the PMP metric
• PM reduction research now concentrating on GDI
• Real-world driving emissions increasingly important
