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On the morphology of internal combustion engine exhaust particles
Pergamon
J. Aerosol Sci., Vol. 28, Suppl. 1, pp. $43-$44, 1997 ©1997 Elsevier Science Ltd. All fights reserved Printed in Great Britain PII:S0021-8502(97)00068-2 0021-8502/97 $17.00+0.00
ON THE MORPHOLOGY OF INTERNAL COMBUSTION ENGINE EXHAUST PARTICLES G. Skillas 1, Stefan Kiinzel 1, Heinz Burtscher 2 , Urs Baltensperger 3, Konstantin Siegmann 1 1Federal Institute of Technology (ETH), Solid State Physics, CH-8093 Ziirich, Switzerland ~HTL Brugg-Windisch, CH-5200 Brugg, Switzerland 3Paul Scherrer Institute, CH- 5232 Villigen, Switzerland
KEYWORDS fractal-like dimension, diesel, spark ignition, soot, inertial impaction, low-pressure impactor METHODS To conduct the measurements, a diesel electrical power generator from Yamaha (Model EDA3000, manufactured in 1987) was used. Loads up to 3.3 k W could be attained. The exhaust aerosol was diluted with a dilution ratio of about 1:10 with dry, heated air. In order to avoid restructuring due to the presence of water, as described by Weingartner et al. (1997), the diluted aerosol was dried in a diffusion dryer while it still had a temperature well over 400 K, (see Fig. ). To eliminate the diesel motor pulsations a membrane pump and in series a 5 l container were used. The flow out of the container was 2 l / m i n . Next, the particles passed a thermodenuder operating at 700 K. The thermodenuder was used selectively, to measure the difference between normal and denuded particles. In order to select a certain mobility, after ensuring that the relative humidity did not exceed 60%, the aerosol particles were led through a diffusion charger and a differential mobility analyser (DMA). The aerosol flow out of the DMA was split into two flows, one leading to an inertial low-pressure impactor, the latter to a reference electrometer. The low-pressure impactor is described extensively in de la Mora et al. (1990), and Schleicher et al. (1995).
Generator~ ~
Filter ~Denuder
Diluti°n~D~ermon~r~o DAQ Impactor ~ Pump~ ~ Cha~er / ~Mobility -- Electrometer ~ r-----selectio n Figure h The experimental setup. Similar measurements were done with a spark-ignition engine power generator from Yamaha (Model EF600, manufactured in 1985) with a nominal power of 600 W. A constant load of 300 W was selected. Both leaded and unleaded gasoline was used. The only difference to the diesel engine setup was that an agglomeration chamber was installed before the thermodenuder, so that the original particles, as well as particles which additional time for agglomeration could be studied. The fractal-like dimension of the soot particles was measured allowing or not allowing for additional agglomeration time. From the impaction data the masses of the particles were computed. The fractal-like dimension is equal to the slope of the Mp(Dm) curve in a double logarithmic plot. This way Fig. 2 was obtained for the single charged particles (55-170 n m diameters) of diesel soot. Similar plots were obtained for the double charged particles of the diesel engine. There was no difference in the fractal-like dimensions of leaded and unleaded gasoline used by the s.i. engine, although an increase in the density of up to a factor of 2 for the thermodenuded particles was measured.
$43
$44
Abstracts of the 1997 European Aerosol Conference
3.00
•
.
,
.
X NTD, single charged particles • TD,single charged particles ,
•
,
,
,
.
i2 57"2. . 5 0 ~ -' Guide i tothe
eye
2.00 0.00
I
I
I
t
/
0.50
1.00
1.50
2.00
2.50
3.00
D i e s e l e n g i n e Load [kW]
Figure 2: The dependence of the fractal-like dimension on the engine load for single charged particles. NTD : without the thermodenuder; TD : with the thermodenuder.
RESULTS As can be seen from Fig. 2 the fractal-like dimension is close to 3 for low engine loads and decreases with increasing load. The first diesel soot particles agglomerate ballistically since their mean free path is much larger than their diameters. This results in compact aggregates with fractal-like dimensions (dqI) close to 3.0. Weingartner et al. (1997) presented transmission electron microscopy graphs of soot particles from the same diesel engine, which also showed a compact structure at low loads corroborating this result of high dql. As the aerosol temperature decreases and the agglomerate size increases, diffusion limited aggregation becomes important. High particle concentrations mean a faster evolution of the particle distribution, resulting in an earlier formation of aggregates through cluster-cluster aggregation while not so volatile compounds begin condensing onto the aerosol particles. This condensation partially offsets the drop of the fraetal-like dimension due to the diffusion limited aggregation. At high loads the fractal-like dimension is close to the theoretical minimum that can be achieved by the low-pressure impactor in the free molecular regime (i.e. dq! = 2.0), indicating that cluster-cluster aggregation dominates at high engine loads. The s.i. engine measurements suggest that the soot particles have a hard core of black carbon, which is immersed in higly volatile material. The core maintains a low fractal-like dimension of 2.2, being surounded by highly volatile hydrocarbons. This despite the fact that soot particles in an atmosphere with a relative humidity over 60 % restructure and their fractal-like dimension increases (Weingartner et al. 1997) and a similar behaviour would be expected for organic compounds. ACKNOWLEDGEMENTS I want to thank Prof. H.C. Siegmann for giving us the opportunity to work in his group and to use its recources, as well as P. Cohn and L. Scherrer for their help and advice concerning the DAQ units and the various mechanical devices used in the experiments. REFERENCES de la Mora, J. F., Nagaraja, R. and McMurry, P. H. (1990) Inertial impaction of fine particles at moderate Reynolds numbers and in the transonic regime with a thin-plate orifice nozzle, J. Aerosol Sci. 21, 889-909. Schleicher, B., Kiinzel, S. and Burtscher, H. (1995) In situ measurement of size and density of submicron aerosol particles, J. Appl. Phys. 78, 4416-4422. Weingartner, E., Burtscher, H. and BMtensperger, U. (1997) Hygroscopic properties of carbon and diesel soot particles Atmos. Environ. 31, in press.
J. Aerosol Sci., Vol. 28, Suppl. 1, pp. $43-$44, 1997 ©1997 Elsevier Science Ltd. All fights reserved Printed in Great Britain PII:S0021-8502(97)00068-2 0021-8502/97 $17.00+0.00
ON THE MORPHOLOGY OF INTERNAL COMBUSTION ENGINE EXHAUST PARTICLES G. Skillas 1, Stefan Kiinzel 1, Heinz Burtscher 2 , Urs Baltensperger 3, Konstantin Siegmann 1 1Federal Institute of Technology (ETH), Solid State Physics, CH-8093 Ziirich, Switzerland ~HTL Brugg-Windisch, CH-5200 Brugg, Switzerland 3Paul Scherrer Institute, CH- 5232 Villigen, Switzerland
KEYWORDS fractal-like dimension, diesel, spark ignition, soot, inertial impaction, low-pressure impactor METHODS To conduct the measurements, a diesel electrical power generator from Yamaha (Model EDA3000, manufactured in 1987) was used. Loads up to 3.3 k W could be attained. The exhaust aerosol was diluted with a dilution ratio of about 1:10 with dry, heated air. In order to avoid restructuring due to the presence of water, as described by Weingartner et al. (1997), the diluted aerosol was dried in a diffusion dryer while it still had a temperature well over 400 K, (see Fig. ). To eliminate the diesel motor pulsations a membrane pump and in series a 5 l container were used. The flow out of the container was 2 l / m i n . Next, the particles passed a thermodenuder operating at 700 K. The thermodenuder was used selectively, to measure the difference between normal and denuded particles. In order to select a certain mobility, after ensuring that the relative humidity did not exceed 60%, the aerosol particles were led through a diffusion charger and a differential mobility analyser (DMA). The aerosol flow out of the DMA was split into two flows, one leading to an inertial low-pressure impactor, the latter to a reference electrometer. The low-pressure impactor is described extensively in de la Mora et al. (1990), and Schleicher et al. (1995).
Generator~ ~
Filter ~Denuder
Diluti°n~D~ermon~r~o DAQ Impactor ~ Pump~ ~ Cha~er / ~Mobility -- Electrometer ~ r-----selectio n Figure h The experimental setup. Similar measurements were done with a spark-ignition engine power generator from Yamaha (Model EF600, manufactured in 1985) with a nominal power of 600 W. A constant load of 300 W was selected. Both leaded and unleaded gasoline was used. The only difference to the diesel engine setup was that an agglomeration chamber was installed before the thermodenuder, so that the original particles, as well as particles which additional time for agglomeration could be studied. The fractal-like dimension of the soot particles was measured allowing or not allowing for additional agglomeration time. From the impaction data the masses of the particles were computed. The fractal-like dimension is equal to the slope of the Mp(Dm) curve in a double logarithmic plot. This way Fig. 2 was obtained for the single charged particles (55-170 n m diameters) of diesel soot. Similar plots were obtained for the double charged particles of the diesel engine. There was no difference in the fractal-like dimensions of leaded and unleaded gasoline used by the s.i. engine, although an increase in the density of up to a factor of 2 for the thermodenuded particles was measured.
$43
$44
Abstracts of the 1997 European Aerosol Conference
3.00
•
.
,
.
X NTD, single charged particles • TD,single charged particles ,
•
,
,
,
.
i2 57"2. . 5 0 ~ -' Guide i tothe
eye
2.00 0.00
I
I
I
t
/
0.50
1.00
1.50
2.00
2.50
3.00
D i e s e l e n g i n e Load [kW]
Figure 2: The dependence of the fractal-like dimension on the engine load for single charged particles. NTD : without the thermodenuder; TD : with the thermodenuder.
RESULTS As can be seen from Fig. 2 the fractal-like dimension is close to 3 for low engine loads and decreases with increasing load. The first diesel soot particles agglomerate ballistically since their mean free path is much larger than their diameters. This results in compact aggregates with fractal-like dimensions (dqI) close to 3.0. Weingartner et al. (1997) presented transmission electron microscopy graphs of soot particles from the same diesel engine, which also showed a compact structure at low loads corroborating this result of high dql. As the aerosol temperature decreases and the agglomerate size increases, diffusion limited aggregation becomes important. High particle concentrations mean a faster evolution of the particle distribution, resulting in an earlier formation of aggregates through cluster-cluster aggregation while not so volatile compounds begin condensing onto the aerosol particles. This condensation partially offsets the drop of the fraetal-like dimension due to the diffusion limited aggregation. At high loads the fractal-like dimension is close to the theoretical minimum that can be achieved by the low-pressure impactor in the free molecular regime (i.e. dq! = 2.0), indicating that cluster-cluster aggregation dominates at high engine loads. The s.i. engine measurements suggest that the soot particles have a hard core of black carbon, which is immersed in higly volatile material. The core maintains a low fractal-like dimension of 2.2, being surounded by highly volatile hydrocarbons. This despite the fact that soot particles in an atmosphere with a relative humidity over 60 % restructure and their fractal-like dimension increases (Weingartner et al. 1997) and a similar behaviour would be expected for organic compounds. ACKNOWLEDGEMENTS I want to thank Prof. H.C. Siegmann for giving us the opportunity to work in his group and to use its recources, as well as P. Cohn and L. Scherrer for their help and advice concerning the DAQ units and the various mechanical devices used in the experiments. REFERENCES de la Mora, J. F., Nagaraja, R. and McMurry, P. H. (1990) Inertial impaction of fine particles at moderate Reynolds numbers and in the transonic regime with a thin-plate orifice nozzle, J. Aerosol Sci. 21, 889-909. Schleicher, B., Kiinzel, S. and Burtscher, H. (1995) In situ measurement of size and density of submicron aerosol particles, J. Appl. Phys. 78, 4416-4422. Weingartner, E., Burtscher, H. and BMtensperger, U. (1997) Hygroscopic properties of carbon and diesel soot particles Atmos. Environ. 31, in press.