Environment International, Vol. 6, pp. 423-426, 1981 0160-4120/81/010423-04502.00/0 Printed in the USA. All rights reserved. Copyright © Pergamon Press Ltd.

PARTICLE SIZE MEASUREMENTS OF AUTOMOTIVE DIESEL EMISSIONS

Joseph D. McCain Southern Research Institute, 2000 Ninth Avenue South, Birmingham, Alabama 35205, USA

Dennis C. Drehmel Industrial Environmental Research Laboratory, Environmental Protection Agency, Research Triangle Park, North Carolina 27711, USA

The automotive diesel engine has long been acknowledged as being "dirtier" than the spark ignition engine and its particulate emissions may be carcinogenic. Possible solutions to the diesel emission prob- lem are combustion modification or aftertreatment devices. Selection of candidate aftertreatment devices requires knowledge of the physical and chemical properties of the particles, including particle mor- phology, size distribution, mass concentration and emission rates in the exhaust gas stream. The study reported here represents the first of a series of experiments designed to characterize the exhaust emissions and test various aftertreatment devices. This paper deals only with the particulate characterization phase of the program. Results of size distribution, particle concentration and mass emission rate measurements for a 5.71 displacement Oldsmobile diesel engine are given for a variety of engine operating conditions.

In t roduct ion

The federally mandated fuel economy standards for passenger automobiles have resulted in considerable im- petus being given to the introduction of substantial numbers of diesel powered automobiles into the passen- ger car fleet. The diesel engine has long been acknowl- edged as being "dirtier" than the spark ignition gasoline engine (by factors of 30 to 50 in particulate emissions). The diesel particulate emissions are primarily car- bonaceous, but 10°70 to 50% by weight of the material is adsorbed higher molecular weight organics, a signifi- cant port ion of which may be polycyclic aromatics (Blacker, 1978). Preliminary results of Ames microbial mutagenicity bioassay tests have indicated the possi- bility that these particulates may be carcinogenic.

Possible solutions to the diesel particulate problem are combust ion modification or the use of aftertreat- ment devices in the exhaust gas stream to collect and /or render the material innocuous. Such treatment may be mandatory if the emissions do prove to represent a significant carcinogenic risk. Selection of candidate aftertreatment devices requires knowledge of the chemi- cal and physical properties of the particles. These in- clude particle morphology, particle size distribution, bulk densities of the collected material, and particulate

mass concentration and emission rates in the exhaust gas stream. Because the organic fraction of the particles appears to be adsorbed on the surfaces of graphitic car- bon base particles, the temperature history of the gas stream may be important . I f the sorption process takes place at elevated temperatures, then collection of the particulate at the normal, relatively hot, exhaust gas temperatures may be sufficient. However, if the sorp- tion takes place only during and after cooling of the exhaust stream to near ambient conditions, the hot par- ticle collection will not result in the removal of the organic fraction.

The study reported here represents the first of a planned series of experiments to characterize the exhaust emissions f rom the point of view of aftertreatment ex- haust gas cleanup and to collect samples for bioassays to determine whether the biological effects of particles col- lected at exhaust line temperatures are the same as those collected after dilution and cooling by ambient air.

Test Program

The tests described here were performed November 27 through December 1, 1978, at a U.S. E P A facility located at Research Triangle Park, North Carolina. A

423

424 Joseph D. McCain and Dennis C. Drehmel

1979 Oldsmobile 88 with a 5.7 ID diesel engine was operated on a Burke E. Porter No. 1059 Chassis Dyna- mometer. The dynamometer was programmed to emulate the Clayton roadload curve for waterbrake dynamometers used for vehicle certification. Test condi- tions included the 13 min. Fuel Economy Test (FET) combined city-highway test cycle, 97 km/h highway cruise, 56 km/h highway cruise, and 56 km/h no load conditions. However, conditions were not equivalent to those required by EPA for vehicle certification and the test results should not be compared to those acquired by official certification methods and conditions.

Sampling and measurement methods included An- dersen cascade impactors, conventional filtration tech- niques followed by condensers and organic sorbent traps, optical single particle counters, electrical mobility methods, and diffusional methods. All samples were taken directly from a modified exhaust pipe which was run out from under the chassis and alongside the passenger side of the automobile to permit reasonable access to the exhaust stream.

Andersen Model III cascade impactors with glass fiber impaction substrates and backup filters were used to obtain total particulate loadings and particle size distributions on a mass basis over the size range from about 0.4 #m to 5 /zm on an aerodynamic basis. The impactors were operated in an oven close-coupled to the exhaust pipe. During runs at a steady engine load (56 km/h and 97 km/h), the oven was maintained at the same temperature as the exhaust gas temperature at the sampling point. During the 13 min FET cycle testing, the impactors and ovens were maintained at about the average exhaust gas temperature for the cycle, 175 °C.

In addition to the cascade impactors, a Thermo- systems Model 3030 Electrical Aerosol Analyzer (EAA) was used to determine concentrations and size distribu- tions of particles in the size range of 0.01 /=m to 0.5 /zm. Particle concentrations ranging from 0.3 /~m to

2.5 #m were monitored using a Royco Model 225 opti- cal particle counter. The Southern Research Institute SEDS III sample extraction, conditioning, and dilution system was used as an interface between the exhaust system and the EAA and particle counter. This system provides a mechanism for the removal of condensible vapors from the sample gas stream at elevated tempera- tures followed by quantitative dilution to particle con- centrations within the operating ranges of the measure- ment instruments.

Fig. l is a diagrammatic sketch of the layout of the exhaust system and measurement instrumentation dur- ing the tests.

The vehicle operating conditions were selected to pro- vide samples collected at elevated exhaust gas tempera- tures prior to cooling for the same engine cycle as the very large (10 kg) sample collected for bioassay work. This 10 kg bioassay sample was being collected using the FET test cycle using a standard "constant volume"

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automatic dilution tunnel. The hot samples collected were to be used for Ames tests to provide some indica- tion of the relative mutagenicity of material collected at the exhaust line temperatures and material collected after cooling and dilution. This information is intended to provide some insight into whether hot collection of the particles will remove the carcinogenic component of the exhaust. In the program as originally conceived, samples were to be taken at a number of points between the exhaust manifold and the tailpipe to provide samples collected over a range of temperatures and engine loads. These would have provided information on changes in particle size distribution and composition as the exhaust gases were cooled. Time limitations in preparing for the tests rendered it impossible to carry out the proposed plan. However, it was found that a considerable swing in gas temperature did occur with changes in engine load. But it is not possible to differen- tiate between engine load/speed induced and tempera- ture induced concentration and composition changes in the data obtained during this test.

Overall particulate loadings, engine gas flows, and sampling temperatures for the cascade impactor sam- ples are given in Table 1. Particle size distributions for the various conditions are given in Figs. 2, 3, 4 and 5. Each figure in this series contains a plot of cumulative percentage smaller than the indicated diameter versus diameter from the impactor data alone and that obtained by integrating the distributions from the electrical aerosol analyzer up to 0.5 /zm and continuing the inte-

Particle size measurements of automotive diesel emissions

Table 1. Results of Cascade impactor sampling.

Average exhaust Aerodynamic mass Average exhaust temperature at Particulate median particle

Operating volume flowrate sampling location loading diameter mode (m3/s) (°C) (mg/m 3) (#m)

FET cycle 0.051 177 68 0.26 97 kmph 0.057 218 55 0.54 56 kmph 0.033 149 45 0.46

(with load) 56 kmph (Not available) 129 39 0.33

(no load)

425

gration from 0.5 #m to 10 #m with the impactor data. A particle density of 1.0 g/cm 3 was assumed for the in- tegrations of the EAA data. The overall size distribu- tions from 0.01 #m to 10 #m obtained in this fashion agree very well with those obtained from the impactors alone.

The variability in particulate concentrations through the FET cycle is illustrated in Fig. 6. This shows particle concentrations v e r s u s time in three particle size intervals through several test cycles. These data were obtained us- ing the optical particle counter.

The particulates collected at exhaust gas temperatures were found to be approximately 15% by mass organics. The results of the biotesting of the samples from the im-

pactors, filters, and organic vapor traps will be reported elsewhere.

Conc lus i ons

Typical particulate concentrations at exhaust line conditions for the Oldsmobile 5.7 ID diesel engine were found to be about 50 mg/m 3. Aerodynamic mass me- dian diameters were about 0,3 to 0.5 /zm with larger median diameters being obtained from higher engine speed/load conditions under steady state operating con- ditions. The results reported here are qualitatively similar in size distribution to those found by other in- vestigators in measurements of emissions from heavy

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426 Joseph D. McCain and Dennis C. Drehmel

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duty diesel engines insofar as the impactor data are con- cerned (Lipkea et al., 1978; Springer and Stahman, 1977).

R e f e r e n c e s

Blacker, S. M. (1978) EPA program to assess the public health sig- nificance of diesel emissions, Air Pol. Control Assoc. 28, 769

Lipkea, W. H., Johnson, J. H. and Vuk, C. T. (1978) The physical and chemical character of diesel particulate emissions: Measure- ment technique and fundamental considerations. Society of Automotive Engineers Paper #780108, presented at the Society of Automotive Engineers Congress and Exposition, Detroit, MI, February 27-March 3, 1978.

Springer, K., and Stahman, R. (1977) Removal of exhaust particulate from a Mercedes 300D diesel car. Society of Automotive Engineers Paper #770716, presented at the Society of Automotive Engineers Off-Highway Vehlde Meeting and Exhibition, Mecca, Milwaukee, WI, September 12-15, 1977.