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Calibration of Fast Electrical Mobility Spectrometers for Engine Particulate

Measurement

ETH 2007

Kingsley Reavell & Jonathan SymondsCambustion Ltd, Cambridge UK

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Content of Talk

Principle of DMS type Fast Particulate Size SpectrometerCalibration Procedure– Gain (concentration)– Size

Relative charging of simple & agglomerate particlesEffect on DMS calibrationApplication of dual agglomerate : spherical calibrations with automatic mode identificationComparison of mode identification with proposed PMP R83 volatileparticle remover + particle counterConclusions

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DMS Principle of Operation

Unipolar diffusion chargerElectrometer detectionSizing by charge : drag ratio - electrical mobility

‘Fast Response Classification of Fine Aerosols with a Differential Mobility Spectrometer’; Reavell, K. Proc. AGM Aerosol Soc. UK. 2002

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DMS Calibration Procedure

Gain CalibrationComparison with aerosol electrometerSingle-charged particles of desired size selected in DMA– Requires small aerosol – renucleated H2SO4 and NaCl

Traceable to calibration of electrometer & flowmeterBetter than CPC standard– only reliable in count mode, ∴differential dilution required, with uncertainty

Size CalibrationSmall sizes – calibration against DMA (mobility standard)Larger sizes (prone to multiple charging issues in DMA) – certified PSL spheres– requires good size resolution

Information incorporated into transfer function used in deconvolution of electrometer currents

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Charging of Agglomerate Particles

Size classification by charge : drag ratioConcentration measurement by electrical current

Measurement sensitive to charge distribution of particles.

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Charge & Size Measurement

Vehicle (steady state)

Feedgas

DPF

DMS Sample Head DMS 500

HR diluter OFF

4:1 Dilution Air

Heated Line @ 1 Atm

3080 DMA

Flow Meter

HEPA Filter

1 slpm

7 slpm

8 slpm

Vehicle (steady state)

Feedgas

DPF

DMS Sample Head DMS 500

HR diluter OFF

4:1 Dilution Air

Heated Line @ 1 Atm

3080 DMA

Flow Meter

HEPA Filter

1 slpm

7 slpm

8 slpm

2. Diesel Agglomerates

particlecharge

sizedistribution

Comp. Air / HEPA

Nebuliser & Dryer

Saturator & Condenser

DMS500

chargerEjector Pump

Peristaltic Pump

SMPS

PAMSLG

OR

Buffer Bag

DMA SMPS

HEPA Spill

Comp. Air / HEPA

Nebuliser & Dryer

Saturator & Condenser

DMS500

chargerEjector Pump

Peristaltic Pump

SMPS

PAMSLG

OR

Buffer Bag

DMA SMPS

HEPA Spill

1. Simple Particles

particlecharge ‘Charge Distribution Produced By Unipolar Diffusion

Charging of Fine Aerosols’; Reavell, Symonds & Biskos. Conf. AAAR 2004

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Unipolar Diffusion Charging Comparison- simple particles vs. agglomerates

DMS500 Mean Particle Charge

0

5

10

15

20

25

30

35

0 50 100 150 200 250 300 350 400 450 500

mobility diameter (nm)

mea

n ch

arge

NaCl / ejector pumpDEHS / peristaltic pumpDiesel, 4th Gear 70kphDiesel, 5th Gear 70kph

~dp1.06

~dp1.25

combustion agglomerates

simple particles

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DMS:DMA sizing comparison

0

50

100

150

200

0 50 100 150 200 250 300Dme (DMA) / nm

Dem

e (D

MS)

/ nm

Diesel: 4th gear, 70 kph

Diesel: 5th gear, 70 kph

Mini-CAST Aerosol

Similar charging behaviour for both engine loadsCombustion aerosol generator also similar Possible small differences at very top of size range

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-50.00%

0.00%

50.00%

100.00%

150.00%

200.00%

250.00%

10 60 110 160 210 260 310

Dp / nm

% d

iffer

eren

ce

Gain Difference (versus electrometer)Size Difference (versus DMA)

Difference between spherical & agglomerate particles causes errors in measurements of larger agglomerate particles made with a calibration for spherical ones

– due to coupling of size & concentration measurements

Effect of Agglomerate Charging on Instrument Calibration

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New Data Processing Facility- Automatic Lognormal Mode Identification

Mode identification software replaces continuous spectrum with 6 parameters:

Bayesian algorithm identifies modes significantly above noise base of instrument, and identifies whether accumulation or nucleation mode.Reduces noise in measurements by increasing data redundancy (6 vs 38 DoF)Removes cross sensitivity introduced by crude size cut-off methodsImproves spectral resolution – also improves calibration accuracy with PSL spheresAlgorithm operates directly on electrometer current data:

– Different transfer functions can be used for the two modes

Nucleation AccumulationGMD GSD N/cc GMD GSD conc

nucleation accumulation

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Real-time Mode Identification- transient vehicle test

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Validation of Accumulation Particle Number Measurement

Peugeot 406 HDi 2.2l common rail diesel vehicle (no DPF, with DOC).Compare DMS500 accumulation mode number concentration with output from PMP-like CPC + VPR system

– CPC + VPR equivalent system measurements corrected with true solid particle penetration measured with NaCl (not in proposed standard)

– CPC TSI 3022 (ie. non PMP-compliant lower size threshold)DMS500 accumulation mode calibrated with miniCAST aerosol, (DMA & Electrometer)Tests under no load:

– Ejector diluter in tailpipe, DF ~ 4.7– Idle, fast idle, transient and high-rev (U.K. “MOT test”) conditions

– CPC “REF” used post PND1 to measure nucleation mode concentrationTests under load:

– Chassis dynamometer– CVS tunnel used– New European Drive Cycles

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PMP equivalent dilution system

ET effectiveness measured at 99.7% with H2SO4 with T1 = 50°C

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No Load Transient Cycle:Real-time comparison DMS vs CPC + VPR

DMS & CPC Measurement of Nucleation & Accumulation Modes

0.0E+00

1.0E+06

2.0E+06

3.0E+06

4.0E+06

5.0E+06

6.0E+06

7.0E+06

8.0E+06

9.0E+06

1.0E+07

0 200 400 600 800 1000 1200

time (s)

conc

entr

atio

n (N

/cc)

-200

-150

-100

-50

0

50

engi

ne a

irflo

w (g

/s)

Conc Accumulation Mode DMS Conc CPC + VPR Conc Nucleation Mode DMS Conc Nucleation CPC MAF g/s

DMS & CPC Measurement of Nucleation & Accumulation Modes

0.0E+00

1.0E+06

2.0E+06

3.0E+06

4.0E+06

5.0E+06

6.0E+06

7.0E+06

8.0E+06

9.0E+06

1.0E+07

0 200 400 600 800 1000 1200

time (s)

conc

entr

atio

n (N

/cc)

-200

-150

-100

-50

0

50

engi

ne a

irflo

w (g

/s)

Conc Accumulation Mode DMS Conc CPC + VPR MAF g/s

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No Load Total Emissions Comparison:DMS Accumulation Mode vs. CPC + VPR

Solid particle number data corrected to pre-PND1 and averaged per second.

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

Transie

nts Idle

Transie

ntsCold

Start

Warm up

Fast Id

le

Fast Id

le40

00 rp

mTran

sients

"MOT"

N/c

c/s

Ave Acc N/cc CPC + VPR

Ave DMS Spherical Calibration

Ave Acc N/cc DMS Agglomerate Cal

mean DMS accumulation ~ CPC +9%mean DMS spherical cal ~ CPC +44%

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Loaded Transient Tests:NEDC on Dynamometer (1)

0.E+00

1.E+07

2.E+07

3.E+07

4.E+07

5.E+07

6.E+07

0 200 400 600 800 1000 1200

Time (s)

Con

c (N

/cc)

0

20

40

60

80

100

120

140

Nucleation Mode (DMS)Accumulation Mode (DMS)Accumulation Mode (PMP)Road speed (kph)

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NEDC on dynamometer: Repeats

0

1E+13

2E+13

3E+13

4E+13

5E+13

6E+13

7E+13

8E+13

9E+13

1 2 3

Test

N /

km

N / km DMSN / km PMP

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NEDC with DPF Fitted

Same vehicle with DPF fittedDirect sampling post-DPF with DMS500Newly regenerated DPFTotal particle number from DMS would fail proposed standardWith automatic mode identification, accumulation mode emissions correctly resolved below limit. 0.0E+00

5.0E+10

1.0E+11

1.5E+11

2.0E+11

2.5E+11

3.0E+11

3.5E+11

4.0E+11

0 200 400 600 800 1000 1200Time (s)

Num

ber o

f par

ticle

s pe

r sec

ond

0

20

40

60

80

100

120

140

Speed (kph)

Nuc Mode N/sAcc Mode N/sSpeed (kph)

Acc Mode N / km Nuc Mode N / km Total N / km Proposed PMP limit N / km

1.9×1011 1.4×1012 1.6×1012 5.0×1011

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Conclusions

A traceable calibration procedure based on comparison with an aerosol electrometer for gain and PSL spheres for size calibration is described for DMS type instruments.Combustion agglomerates are more highly charged in the DMS unipolarcharger ( ~ dp

1.25) than simple particles ( ~ dp1.06).

This affects both the size, and particularly, concentration measurement of agglomerate particles in electrical mobility based sizing instruments.A calibration for agglomerate particles which can be automatically applied to just the accumulation mode of an exhaust aerosol is demonstrated and compared with a PMP-like CPC + VPR measurement system.The average difference between the CPC+VPR measurement and agglomerate calibrated DMS is ~9% on engine exhaust aerosols.The automatic mode identification allows the DMS to measure accumulation mode levels in the presence of significant nucleation concentrations.There are significant differences between nucleation concentrations measured with the DMS and by differencing two CPCs.

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Acknowledgements

Thanks to Bruce Campbell, Roy Stubbs and Paul Davies for the dynamometer testing.

And to the organising committee of the conference.

Questions?