Physical and Chemical Characteristicsof Particulates in Spark Ignition Engine Exhaust John T. Ganley and George S. Springer'
Department of Mechanical Engineering, T h e University of Michigan, A n n Arbor, Mich. 481 04
The formation and emission of particulates in the exhaust gas of a 350-ir1.~Chevrolet V-8 spark ignition engine were investigated. The engine was mounted on a dynamometer and operated a t constant speed and load. Sampling of the particulate matter was preformed a t points along a simulated exhaust system. Both leaded (Indolene HO 30) and unleaded (Indolene HO 0) fuels were used, and the weight concentration and size distribution of the particles were measured over a wide range of exhaust gas temperatures (90-830°F). Experiments were also performed evaluating the effects of engine speed, air-fuel ratio and spark timing on the amount of particulates emitted with Indolene HO 0 fuel. In addition to the physical characteristics of the particles, the chemical compositions of the particles were also determined by X-ray diffraction and with an electron microprobe. The chemical compositions were evaluated as a function of both exhaust gas temperature and particle size. Finally, an analytical model was developed for describing the dependence of weight concentration on exhaust gas temperature Particulates emitted from spark ignition engines may contribute to air pollution problems by influencing the meteorology of the environment, by introducing into the atmosphere substances deleterious to health, and by hindering attempts to control gaseous emissions. Effective control of particulate emissions requires a knowledge of the effects of all those parameters which influence the formation and emission of particles. In this investigation the physical characteristics (weight concentration and size) of the particles were studied as a function of exhaust gas temperature, fuel composition, air-fuel ratio, spark timing, and speed and load. Furthermore, the chemical composition of the particles was examined as a function of exhaust gas temperature, fuel lead content, and particle size. The weight concentration, size distribution, and chemical composition of the emitted particulates have been investigated widely in the past (e.g., see Habibi, 1973; Springer, 1973). Almost all of the previous data were obtained at approximately room temperatures. However, it has been shown that the exhaust gas temperature plays a very significant role in the particulate formation process (Moran et al. 1971, Sampson and Springer, 1972). Therefore, in this study, particular attention was paid to the effects of the exhaust gas temperature in the range 90800°F. Experimental Apparatus and Procedure The apparatus and procedures were essentially the same as those used by Sampson and Springer (1972) and, therefore, they are not described here in detail. Only a brief summary is given to indicate the changes made. A 1970 350-in.3 250-hp Chevrolet V-8 production engine, mounted on a dynamometer, was used in all tests. Tests were conducted with both leaded (Indolene HO 30) and unleaded (Indolene HO 0, clear) fuels. The physical and chemical properties of the fuels are given by Ganley and T o whom correspondence should be addressed. 340
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Springer (1973). A simulated exhaust system was connected to the engine as shown in Figure 1. Although the design of this exhaust system differed somewhat from that of a production exhaust system, its dimensions, temperatures, and pressures were similar to those observed in the exhaust system of an actual automobile (Ganley and Springer, 1973). Two particulate collection units were employed (Figure 2). Unit I was used to collect particles directly from the exhaust stream, while in Unit I1 the exhaust gas was diluted with room air before the particulate matter was collected. It is important to note that in Unit I1 the meaCROSSOVER
EXHAUST HANIFOL
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Figure 1. Arrangement of experimental apparatus Circles designate sampling port and thermocouple locations PROBE FROU
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Figure 2. Schematic of particle collection units (a) Particle collection unit I : (b) particle collection unit I I . Open circles indicate thermocouple locations
DILUTION RATIO,
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Figure 3. Particulate weight concentration as a function of dilution ratio 55 Mph cruise condition, lndolene HO 0 fuel. Sampling temperature 95°F. 0 Data, -- f i t to data, concentration in the case when no Darticies form after dilution
sured weight concentration of the particulate depends on the dilution ratio-i.e., on the ratio of the total mass of mixture to the mass of exhaust gas flowing through the sampler (Figure 3). If particles would not form subsequent to dilution, the weight concentration would follow the dotted line in Figure 3. However, particles did form within the sampler as indicated by the data (solid line). In all the present tests, a 9:l dilution ratio was used. The filters and the Andersen stack sampler were used to determine the weight concentration and size distribution of particulates. The shapes of the particles were also examined with a Scanning Electron Microscope. Chemical analysis of particulate matter was performed with an ARL electron microprobe to determine the elemental composition of the particles, and by X-ray diffraction to identify the lead compounds. The majority of the tests were performed at a steady engine speed of 1800 rpm and a constant load of 24.5 bhp (brake horse power). This load is equivalent to the road load for a full size 1970 Chevrolet cruising a t 55 mph, and is referred to as the 55 mph cruise condition. Some tests were also performed a t 40 and 70 mph cruise conditions. Prior to taking data, the engine and exhaust systems were properly conditioned. This was achieved by operating the equipment for a sufficient period of time in a manner which allowed rates of formation, deposition, and reentrainment of particulates to attain values which remained constant throughout the subsequent tests. For details of the procedures employed in the conditionings and in all the tests, the reader is referred to Ganley and Springer (1973). It is noted here that the composition of the exhaust gas was analyzed throughout the tests, but no correlation was observed between the gaseous and particulate emissions.
Results Effect of Fuel Lead Content on Weight Concentration. The measured weight concentration of the particles as a function of exhaust gas temperature is given in Figure 4. When we used Indolene HO 30, there was a large increase in weight concentration as the exhaust gas temperature decreased from 630-470°F, attributable to the condensation of lead salt vapors in the exhaust system. X-ray diffraction analysis showed that an alloy of PbClz and PbClBr was present in the samples at 470°F but not at 640°F. These lead salts result from the addition of scavenging agents (ethylene dichloride and ethylene dibromide) to the fuel. With Indolene HO 0, the weight concen-
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tration was constant at all exhaust gas temperatures above 250°F. At temperatures below 250"F, an increase in weight concentration was observed when using both Indolene HO 0 and Indolene HO 30. Although chemical analysis of this particulate matter was not performed, the increase is probably due to the condensation of high-molecularweight organic compounds present in the exhaust gas. There is evidence to indicate that these compounds result from distillation and chemical reaction of a small portion of the lubricating oil that has found its way into the combustion chamber (McKee et al., 1957, 1960; Dubois et al., 1970; Cukor et al., 1972). The increase in weight concentration between 250" and 90°F is approximately the same for the two fuels (0.35 mgm/sft3) indicating that the presence of the lead does not affect the production of particulates below 250°F. The data of Sampson and Springer (1972) are also included in Figure 4. For Indolene HO 0 the data from both investigations agree very well. Two interesting points may be made when comparing the data obtained with leaded fuels. The first is that the weight concentration level at temperatures below 470°F is higher when Indolene HO 30 is used than when regular pump gasoline is used. The reason is that Indolene HO 30 contains more lead per gallon than the regular pump gasoline (3.0 ml TEL/gal vs. 2.4 ml TEL/gal, respectively). The second point is that the sharp increase in concentration occurs at a higher temperature with pump gasoline than with Indolene HO 30. This may be attributed to the fact that different compounds are formed when the two different gasolines are used. X-ray diffraction analysis of the particulate matter collected in the present study (Indolene HO 30) showed that the particles were composed mainly of PbC1z.PbClBr (at 465°F). The samples collected by Sampson and Springer (regular pump gasoline) were primarily made up of PbClBr and PbO.PbBr2. Above 800"F, the use of Indolene HO 30 results in a slightly higher weight concentration than Indolene HO 0, while the use of regular pump gasoline gives considerably higher values. The particles present at temperatures above -800°F were likely formed in the combustion chamber. This is the case when Iodolene fuels were used because X-ray diffraction analysis of the particulate matter collected at this temperature showed that the particles were mostly carbon. These carbon particles are formed when hydrocarbons are dehydrogenated during the combustion process (Street and Thomas, 1955). With Indolene
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Figure 4. Particulate weight concentration vs. exhaust gas t e m p e r a t u r e at 55 m p h c r u i s e condition 0 , A , Present study 0 . A , data of Sampson and Springer (1972) f i t to data
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1.0 2.0 3.0 FUEL LEA0 CONTENT, ML TEL/GALLON
Figure 5. Particular emission f r o m 3 5 0 C I D Chevrolet V - 8 eng i n e s as a function of fuel lead Content Comparison of experimental data. Exhaust gas at room temperature. Present study: 1800 rpm steady road load: 0 lndolene HO 30, A Indolene HO 0. Moran and Manary (1971): 2250 rpm, steady road load; D lndolene HO 30, 0 lndolene HO 15. eP lndolene HO 0. Sampson and Springer (1972): 1800 rpm, steady road load; 0 regular (leaded fuel) V o l u m e 8 , N u m b e r 4 , April 1974
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HO 30, small amounts of lead, chlorine, and bromine were also observed by the electron microprobe in the samples. These elements are probably present as lead oxyhalide because both PbO .PbBrz and PbO .PbS04 were identified by X-ray diffraction in samples of combustion chamber deposits taken when the engine was dismantled after the tests. The intermittent “tearing off” of bits of these deposits is felt to be the reason behind the higher weight concentration and increased scatter observed with Indolene HO 30. Additional information on the effect of fuel lead content on particulate emissions may be gained by comparing the results of the present study with the results of previous investigations. It is necessary to make these comparisons for the data taken at room temperature since most other investigators did not take data at higher exhaust gas temperatures. The influence of the amount of fuel lead content on particulate emissions can be illustrated by comparing the data of the present study with the data of Sampson and Springer (1972), and Moran and Manary (1970) (Figure 5). Moran and Manary also used a 350-i11.~250-HP Chevrolet V-8 engine, but it was operating at a different condition than in the present study (60 mph vs. 55 mph cruise conditions) and sampling was performed in a large tunnel in which the total exhaust had been collected and diluted. Although the values are slightly different, a similar trend of increased particulate emissions with increased fuel lead content is evident in both investigations. Another parameter of interest is the ratio of the amount of lead emitted from the exhaust system to the amount of lead taken in by the engine. This is usually expressed as a percent of input lead exhausted, and has been measured by several investigators (Hirschler et al., 1957; Habibi, 1970; Ter Haar et al., 1972). Figure 6 presents a summary of the existing data. Note, however, that the values given in this figure are averages only. At any speed there is a large variation in the measured values of percent of lead emitted. The spread in data increases with speed. The data of the present study, and the data of Sampson and Springer (1972) and Ter Haar et al. (1972) are in good agreement. The values given by Ter Haar et al. are average values for a fleet of 26 1963 to 1968 cars. The results given by Hirschler et al. (1957, 1964) and Habibi (1970) are both higher than these values. Hirschler’s data are for older cars which typically had lower rear axle ratios giving higher engine speeds at comparable road speed. This and the less sophisticated sampling procedures possibly ac-
Figure 6 Percent of input lead exhausted vs. speed. Comparison of the result of the present study with results from full size cars operated at road load conditions on chassis dynamometers 0 Present study, 0 Sampson and Springer (1972), Q Hirschler, et al’. (1957), 0 Habibi (1970), vertical lines indicate spread in data, 0 Ter Haar et al. (1972)
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Figure 7. Size distributions of particles as a function of exhaust gas temperature
count for the difference. The reason for the discrepancy with Habibi’s data is not clear. It is also interesting to compute the weight percentage of lead compounds present in the particulate matter. Above 250”F, the difference in particle weight concentration with Indolene HO 30 and Indolene HO 0 fuels is due to the appearance of various lead compounds. In Figure 4 this is manifested by the difference in weight concentration between lines c-d and a-b. Dividing this difference by the total weight concentration at 90°F (point e ) gives the weight percentage of all lead compounds present in the exhaust particles as 66.3%. The results of the X-ray diffraction analysis showed that the major lead compound in the particulates was PbClz-PbClBr. If we assume that all of the lead is in this form, then the weight percent of lead present in the particulates is 45.7%. This value compares very favorably with the value of 37.6-43.370 published by Mueller et al. (1962), whose results are for 3 cars using fuel with 3 ml TEL/gal and operating on a chassis dynamometer at a steady 60-mph road load condition. It must be emphasized that all results presented in the foregoing were obtained a t steady speeds rather than under cyclic operating conditions. The latter condition generally yields 4-6 times higher emission rates than the former one. Nevertheless, the present results show that the ratio of emissions using Indolene HO 30 to those resulting from use of Indolene HO 0 is approximately 3-1 at room temperature. This is comparable to the results of other investigators, even including data taken under cyclic conditions (Habibi, 1970; Ninomiya et al., 1970; Moran et al., 1970; Ter Haar et al., 1972). Effect of Fuel Lead Content on Particle Size Distribution. Measurements of the size distribution of the exhaust particles were made over a wide temperature range (90-830°F) a t the 55-mph cruise condition. With Indolene HO 0, the size distributions lie within a relatively narrow band, and no discernible trend with respect to temperature was observed (Figure 7). The mass median aerodynamic diameter at the center of the band is approximately 1.4 pm. With Indolene HO 30, several size distributions were obtained of which five representative ones are shown in Figure 7. The size distribution is a strong function of exhaust gas temperature. This is clearly seen in Figure 8 where the mass median aerodynamic diameter is plotted vs. exhaust gas temperature for all the size distributions measured. A similar curve is obtained when replotting in the same manner Sampson and Springer’s data (1972). The increase in particle size is due to condensation of vapor on particles flowing in the stream, agglomeration of particles in the stream, and entrainment of particles that have formed and grown on the walls of the exhaust sys-
DISTANCE ALONG EXHAUST SYSTEM, INCHES 260 220 180 140 100
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Figure 8. M a s s m e d i a n aerodynamic diameter vs. exhaust temperature indolene HO 30, 55 mph cruise condition
gas
tem. An order of magnitude analysis of these effects was performed which showed that particle growth is primarily due to vapor condensation. Agglomeration and entrainment are probably secondary effects (Ganley and Springer, 1973). The decrease in mass median diameter at temperatures below 568°F appears to be caused by the strong effect of particle size on the particle deposition rate. Schwendiman and Postma (1961) presented a correlation for turbulent deposition in pipes. Applying their correlation to the flow in the present exhaust system. we find that in a distance of 60 in. (the approximate distance between the 568" and 470°F samqling points), about 75% of the 10-pm particles, but only about 0.5% of the particles less than 1 pm, are deposited on the walls. This indicates that the walls of the exhaust system "trap" the larger particles and let the fine particles pass. T o further demonstrate this effect, the mass median diameters of the particles were calculated using the deposition data of Schwendiman and Postma. The calculated results agree qualitatively with the data (Figure 8). We now turn our attention to the effect of fuel composition on the particle size distribution. Figure 9(a) compares the size distributions at high temperatures. This is, roughly speaking, the size distribution of the particles entering the simulated exhaust system. It is evident that the smallest particles are present when regular pump gasoline is used. Use of Indolene HO 30 gives particles appreciably larger than the pump gasoline but smaller than Indolene HO 0. Notice that these curves were obtained in a virtual-
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