Evaluation of gaseous and particulate emission characteristics of a single cylinder diesel engine

C O M B U S T I O N A N D F L A M E 70:215-224 (1987)

215

Evaluation of Gaseous and Particulate Emission Characteristics of a Single Cylinder Diesel Engine AKIRA OBUCHI, AKIHIKO OHI, HYOGORO AOYAMA, and HIDEO OHUCHI Automotive Exhaust Control Division, Air Pollution Control Department, National Research Institute f o r Pollution and Resources, Onogawa 16-3, Yatabe, Tsukuba, Ibaraki 305, Japan

Emission characteristics of a single cylinder diesel engine with a swirl chamber were investigated. Specific fuel consumption, exhaust gas temperature, specific mass emissions of CO, total hydrocarbon (THC), nitrogen oxides (NOx), and particulates were measured. In addition, contents of carbon, hydrogen, CH2C12 soluble fraction (SOF), aliphatic hydrocarbons, polar organics, benzo(a)pyrene (B(a)P), and low temperature combustion fraction in the particulates, and the burnout temperature of the particulates were analyzed. All these results were represented as contour maps on a speed-torque plane. The emission of CO and the particulates increased steeply near the maximum speed and torque investigated, whereas the NOx emission was almost independent of speed, increasing as the torque decreased. Content of hydrogen, SOF, aliphatic hydrocarbons, and low temperature combustion fraction showed similar patterns, having the maximum value at 1500 rpm at 1/4 load and decreasing toward 2500 rpm at full load. Burnout temperature of the particulate matter ranged from 673 to 707°C and correlated with the carbon content.

INTRODUCTION Although the diesel-powered vehicle is superior in fuel economy, it emits more particulate matter than a gasoline-fueled vehicle and the emission of nitrogen oxides is also becoming a serious environmental problem [1, 2]. In particular, diesel exhaust particulates contain a large amount of soluble organic fraction, some of which is considered to be mutagenic or carcinogenic, such as polycyclic aromatic hydrocarbons (PAH) and their nitro-derivatives [3]. A comprehensive investigation into the diesel-powered vehicle's emission characteristics is needed. In view of the recent increase in diesel-powered vehicles, the Japanese Environment Agency has established new standards for NOx reduction which will be enforced by 1987 and are considering the enforcement of a regulation for the particulate reduction [4]. Diesel exhaust particulates are believed to be produced through pyrolysis of the fuel during local incomplete combustion in the cylinder, whereas nitrogen oxides are produced mainly by oxidation of nitrogen molecules in the air in the high Copyright © 1987 by The Combustion Institute Published by Elsevier Science Publishing Co., Inc. 52 Vanderbilt Avenue, New York, NY 10017

temperature combustion zone. The producing conditions of these two substances are therefore contrary to each other. As a result, a simultaneous reduction of these substances is a difficult problem and the approach using only the optimal control of the combustion conditions or exhaust gas recirculation (EGR) is so far not sufficient [5]. The use of a regenerative exhaust filter trap is effective for the particulate reduction but there remain some problems regarding durability and the regeneration techniques to overcome [6]. In this paper, we shall report the effects of engine speed and torque on the emission rate of various substances and the characteristics of particulates from a single cylinder diesel engine. The purpose of this research is to understand the characteristics of diesel emissions in order to control the exhaust substances, especially the particulate matter.

EXPERIMENTAL The engine used was a single cylinder research diesel engine with a swirl chamber (Toyota Cen-

0010-2180/87/$03.50

216

AKIRA OBUCHI ET AL. TABLE 1

TABLE 3

Test Engine

Engine Operating Condition

Toyota TRE-2DS Swirl type prechamber 90 × 86 0.547 21.2

Model Combustion chamber Bore x stroke (ram) Displacement [1] Compression ratio Valve timing Intake

Open: 14 deg. BTDC Closed: 44 deg. ABDC Open: 51 deg. BBDC Closed: 11 deg. ATDC

Exhaust

tral R & D Lab. Inc., TRE-2DS). The fuel was commercial gas oil satisfying JIS (Japan Industrial Standard) K 2204. The engine and fuel specifications are listed in Tables 1 and 2, respectively. The engine speed was set at 1000, 1500, 2000, 2500, and 3000 rpm. The fuel injection timing for each speed was set where the specific fuel consumption was the minimum. The full load was determined where the smoke density was 40% in opacity. Steady-state operation was performed at 1/4, 2/4, 3/4, and full load at each speed. In Table 3, the torque at each test point is listed. The measurement results at these 20 test points were analyzed as one set with a data analysis program (Fujitsu, AXEL II) and represented in the form of a contour map on a speed-torque plane. The measured items relating to the engine emission characteristics were specific fuel consumption, gas temperature, specific emission of CO, total hydrocarbon (THC), NOx and particulates, contents of carbon, hydrogen, CH2C12 soluble fraction (SOF), aliphatic hydrocarbons, polar organics, benzo(a)pyrene, and low temperature TABLE 2 Test Fuel Specific gravity (15/4°C) Average molecular weight Elemental content (wt%) C H Aromaticity Cetane number

0.8420 224 86.4 13.3 0.13 51

Torque (kg m) Load Speed (rpm)

1/4

2/4

3/4

4/4

1000 1500 2000 25O0 311)00

0.68 0.81 0.75 0.65 0.70

1.36 1.56 1.47 1.33 1.36

2.06 2.31 2.18 1.95 1.99

2.76 3.10 2.90 2.61 2.63

combustion fraction in the particulates, and the burnout temperature of the particulates. A schematic diagram of the system used for the engine operation, exhaust gas analysis, and a sampling of the particulates is shown in Fig. 1. The fuel supply rate was continually measured from the weight change in the fuel vessel on an electrical balance (Mettler, PC 4000). Temperatures at various parts including the exhaust gas were measured with a chromel-alumel thermocouple. The concentrations of CO, THC, and NOx were measured with an automobile exhaust gas analyzer (Horiba, MEXA 2000). In the case of the particulate sampling, the exhaust was diluted in a dilution tunnel (Horiba, DLT-09S) with a 254 mm inner diameter and a 3.0 m effective length. The airflow rate was set at 6.3 m3/min for 1000, 1500, and 2000 rpm and 9.4 m3/min for 2500 and 3000 rpm, resulting in the dilution ratio of 21 to 26 at 1000 rpm, 14 to 17 at 1500 rpm, 12 to 13 at 2000 rpm, 16 to 18 at 2500 rpm, and 12 to 14 at 3000 rpm. Specific particulate mass emission was estimated from the particulate weight sampled on a 47 mm-diameter glass fiber filter reinforced with PTFE (Pallflex Prod., TX40HI20-WW). Fifty to 100 mg of particulate matter collected on a 185 mm-diameter filter made of the same material was submitted to chemical and thermal analyses. Carbon and hydrogen atom contents in the particulates were measured with an elemental analyzer (Carlo Erba, Model 1106). The SOF was obtained by Soxhlet extraction of the particulates for 12 h with CH2C12, and then weight loss of the extracted filter was measured, from which the

217

PARTICULATE EMISSIONS OF A DIESEL ENGINE

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Diesel Englne

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Ic0.,,c.,0x Analyzer1

Fig. 1. Schematic diagram of engine operation and exhaust sampling systems.

content of the SOF was estimated. After filtrating through a 0.45/zm membrane filter (Toyo Roshi, GO45A013A), the extract was subjected to an organic-type analysis with a thin-layer chromatograph with flame ionization detection (Yatron, TH-10). With this analytic method, the extracted organic fraction can be classified into three groups according to the extent developed with n-hexane, which is shown in Fig. 2. The sharp peak at the maximum RF (Rate of Flow) value was attributed to aliphatic hydrocarbons, the other sharp peak remaining at the original spot was attributed to polar organics, and the wide band extending between the above two peaks was attributed to aromatic hydrocarbons. Each group was quantified from the area of the chromatogram. Part of the extract was dissolved into acetonitrile and passed through a pretreatment column (Waters, Sep Pak C-18) to remove any aliphatic compounds and nonsoluble material. Then, the eluate was subjected to the analysis of B(a)P with a high performance liquid chromatograph (pump; Altex, 101A, column; Waters, Radial Pak PAH) with a fluorescence spectrometric detector (Shimadzu, RF 530). The excitation and emission wavelength of the detector were set at 385 and 430 nm, respectively. Details of the analytic procedure for the soluble fraction have been reported elsewhere [7, 8]. Differential scanning calorimetry (DSC) and thermogravimetry (TG) of the particulates were simultaneously conducted with a thermal analyzer

(Rigaku Denki, TG-DSC 8085). Particulates of 0.5 to 0.8 mg were put into a platinum cell with a lid and heated at the rate of 10°C/min from 20 to 800°C under the flow of purified air at 200 ml/ min.

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quinones oliphotics Fu carboxylate$ n-dodecylcorbozoles benzene B(a)P Fig. 2. An example of a thin layer chromatogram with FID for the CH2CI2 extract of diesel exhaust particulates.

218

AKIRA OBUCHI ET AL.

The errors in the gaseous concentration measurements of CO, THC, and NOx were 3%, 10%, and 12% as the coefficient of variation, respectively. The errors in the measurements of the particulate emission rate, contents of the SOF, aliphatic hydrocarbons, polar organics, and benzo(a)pyrene were 4%, 10%, 15%, 9%, and 23%, as the coefficient of variation, respectively [9]. The errors in the analyses of carbon and hydrogen contents were 0.3% and 0.03% as the standard deviation, respectively, and that of the burnout temperature was within 2°C. The extent of errors arising from the application of the data analysis program was not estimated.

3 3

3. 0

0.5 1000

1500

2000

Speed

RESULTS

2500

30OO

( rpm )

Fig. 4. Exhaust gas temperature (*C).

In Figs. 3-8, specific fuel consumption (SFC), exhaust gas temperature, and specific mass emissions of CO, THC, NOx, and particulates are shown. The following aspects of these results are pointed out. SFC had a minimum value of 314 g/kW h at 1000 rpm at full load and a maximum of 621 g/kW h at 3000 rpm at 1/4 load. It decreased with the decrease in the speed and the increase in torque. The exhaust gas temperature ranged from 160°C to 722°C, increasing almost uniformly with the increase in the speed and torque. The minimum CO emission rate was 2.7 g/kW h at 1000 rpm at

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full load and the maximum rate was 15.6 g/kW h at 3000 rpm at full load. There were other peak points at 1000 rpm at 1/4 load and 3000 rpm at 1/4 load. The maximum THC emission rate was 1.10 g/kW h at 1000 rpm at 1/4 load and the minimum rate was 0.11 g/kW h at 2000 rpm at full load. NOx emission rate decreased from 22 to 4.4 g/kW h as the torque increased, showing little dependence on the speed. Particulate emission rate was the minimum near half-load for all the speeds. It increased above 3/4 load, showing a remarkable

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Speed

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250o (rpm)

Fig. 5. CO emission rate (g/kW h).

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PARTICULATE EMISSIONS OF A DIESEL ENGINE 3.3

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Fig. 6. Total hydrocarbon emission rate (g/kW h).

Fig. 8. Particulate emission rate (g/kW h).

increase as the engine operating condition approached 3000 rpm at full load, where the rate was 10.7 g/kW h. In relation to the composition of the particulates, the analytical results for the contents of carbon, hydrogen, SOF, aliphatic hydrocarbons, polar organics, and B(a)P in the particulates are shown in Figs. 9-14. The carbon content ranged from 83 to 95%, having its minimum value at 2500 rpm at 1/4 load and the maximum value at any speed at full load.

The contents of hydrogen, SOF, and the aliphatic hydrocarbons showed a similar tendency. The maximum points were at 1500 rpm at 1/4 load and the minimum points were at 2500 rpm at full load for all cases. As for the polar organics, the content ranged from 0.5 to 4. 1%, with the maximum point shifting 500 rpm upward compared with the above three patterns. B(a)P content had a maximum of 5.9 ppm at 1500 rpm at half-load. It had a large dependence on the speed; that is, the emission steeply decreased as the speed increased from 3.3

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Fig. 7. NOx emission rate (g/kW h).

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Fig. 9. Carbon content in the particulates (%).

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220

AKIRA OBUCHI ET AL. 3.3

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Fig. 10. Hydrogen content in the particulates (%).

Fig. 12. Aliphatic hydrocarbon content in the particulates (%).

1500 to 2000 rpm for lower torque or from 1000 to 1500 rpm for higher torque. The minimum was about 0.01 ppm at 2500 rpm at full load, which is the same as the pattern for hydrogen, SOF, and aliphatic hydrocarbons, but the extent of the variation is much more drastic. In the thermal analysis of the particulates, the TG and DSC curves as shown in Fig. 15 were obtained• In the DSC curve, two exothermic peaks, one small peak around 250-400°C and the

other large peak around 500-700°C were observed. The amount of unburnt residue was below the detection limit (1/~g); that is, its content in the particulates was less than 0.2%. The percentages of the former peak area against the sum of both peak areas, i.e., the ratio of the low temperature combustion peak (RLTC), are shown in Fig. 16. The maximum value was about 6% at 1500 rpm at 1/4 to half-load and the minimum was about 1% at 2500 rpm at full load. This result resembles again the pattern for hydrogen, SOF, and the aliphatic

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Fig. l 1. CH2CI2 soluble fraction content in the particulates

(%).

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3ooo

(rpm)

Fig. 13. Polar organic content in the particulates (%).

PARTICULATE EMISSIONS OF A DIESEL ENGINE 3.3

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Fig. 14. Benzo(a)pyrene content in the particulates (ppm).

DISCUSSION In order to clarify the relationships among the various characteristics of the particulates, a correlation matrix among the analytic results concerning the particulate composition and thermal characteristics was calculated from the original 20 data

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lmcal l

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i

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load. The entire pattern resembles that of the carbon content• The lower the carbon content, the lower the temperature.

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3OOo

Fig. 16. Ratio of the low temperature combustion fraction in the particulates (%).

hydrocarbons so that the peak is attributed to the part of the SOF which is flammable. Ignition temperatures of paraffinic hydrocarbons with more than 7 carbon atoms are 230-240°C [10], which corresponds to the temperature of this peak. The latter peak is attributed mainly to the combustion of carbonized substances• Combustibility of the particulates was characterized by the right end of the peak, i.e., the burnout temperature• The contour map of this value is shown in Fig. 17. The highest temperature was 7070C at 1500 rpm at full load and the lowest was 6730C at 2500 rpm at 1/4

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600

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temperature (°C) Fig. 15. An example of the thermal analysis of diesel exhaust particulates.

222

AKIRA OBUCHI ET AL. 3.3

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(rpm)

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15

20

S O F (%) Fig. 18. Relation between the SOF content and the hydrogen content.

Fig. 17. Burnout temperature of the particulates (°C).

sets, postulating a simple linear relationship between any two sets of results. The coefficients of correlation are shown in Table 4. Content of hydrogen is highly correlated with the content of SOF and the aliphatics. It is also correlated with the polar organics and the RLTC. Figure 18 shows the relation between the SOF content and hydrogen content. Almost a first order relationship was observed and the correlation equation had a constant part; that is, about 0.4% of the hydrogen exists when the SOF content became zero. This constant part of the hydrogen is considered to be the atoms combined to nonsoluble, highly carbonized substances, while the pro-

portional part of hydrogen to be atoms contained in the SOF. From the slope of the line, the hydrogen content in the SOF was estimated to be about 7.8%, which corresponds to organic compounds in which the atomic ratio of hydrogen to carbon is about one. B(a)P content did not correlate to any great extent with any of the other results, even with the SOF. This indicates that the process of the B(a)P formation is different from that of the major components of the SOF. The fact that the B(a)P content is influenced drastically by the engine operating conditions such as the speed, torque, and the fuel properties [11] leads to the speculation that most B(a)P and the same kind of polycyclic

TABLE 4 Correlation Matrix among the Compositional and Thermal Characteristics of the Particulate Matter

Exhaust temp. Carbon (C) Hydrogen (H) SOF Aliphatics (AI.) Polar org. (Pol.) B(a)P Ratio of low temperature combustion peak (RLTC)

C

H

SOF

A1.

Pol.

B(a)P

RLTC

Burnout

0.40

-0.76 -0.68

-0.81 -0.55 0.92

-0.80 -0.42 0.93 0.89

-0.80 -0.68 0.90 0.87 0.89

-0.53 0.14 0.38 0.46 0.53 0.33

-0.71 -0.44 0.85 0.81 0.83 0.76 0.63

0.40 0.80 - 0.67 -0.55 - 0.54 -0.73 0.26 -0.38

PARTICULATE EMISSIONS OF A DIESEL ENGINE aromatic hydrocarbons (PAH) are formed during the combustion process. On the other hand, major components of SOF are suspected to be heavy molecules contained in the fuel and the lubricant oil, unburnt or only moderately changed during the combustion process. Zierock et al. analyzed the contents of PAHs in the particulates from three diesel engines, one of which was a swirl chamber engine with 4 cylinders [12]. They reported that the engine emitted increasingly more PAH-content particulates as the engine speed increased. Our result was greatly different from theirs. In addition to the difference in the number of cylinders, it is likely that these discrepancies in trends are due to fundamental differences in engine design such as airflow, prechamber configuration, injection characteristics, etc., which can alter and even reverse emission trends and levels. It is also possible that there is a contribution due to differences in the manifold and exhaust line configuration; additional PAH reactions are known to occur here and are known to be highly sensitive to local conditions. Burnout temperature is correlated well with the carbon content and also with the content of the polar organics. The relation between the carbon content and the burnout temperature is shown in Fig. 19. Ishiguro and Suzuki investigated the combustion characteristics of the particulates emitted from a 2.2 1 diesel engine with 4 cylinders [13]. They reported the carbon content to be 7891% and the burnout temperature from 610 to 6657C. In their report, a correlation between the carbon content and burnout temperature can be observed, although they correlated the combustibility with the specific surface area of the particulates. In the thermal analysis, the SOF burns, vaporizes, or changes chemically to more stable carbonaceous compounds at an earlier stage of the analysis so that the burnout temperature corresponds to the combustion temperature of the least flammable carbonized material. The variation of the burnout temperature may rise from the structural difference in this final carbon. The higher the carbon content, the more the graphitization is supposed to proceed. It is a well known fact that the more the reactivity of carbon with 02 drops,

223 720

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700

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0 c L..

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680

d~ 660 80

i

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85

90 Carbon

t

95

100

(%)

Fig. 19. Relation between the carbon content and the burnout temperature. the more it is graphitized [14], although it is not clear whether the difference in the reactivity is due to simply the difference in the surface area or a fundamental difference in the surface reactivity [15]. Similarly, the fact that oxygenated carbonaceous compounds generally give disordered carbon [16] is thought to be the cause of the lower burnout temperature for the participants having an initially higher content of the polar organics. CONCLUSION Specific emission rates of various exhaust substances were measured and chemical as well as thermal analyses of the exhaust particulate matter were conducted for a single cylinder diesel engine with a swirl chamber. CO and particulate emission rates increased steeply near the maximum engine speed and torque investigated, whereas the NOx emission rate was higher as the torque became lower, having little dependence on the speed. The content of hydrogen, CH2C12 soluble fraction (SOF), and aliphatic hydrocarbons in the particulates showed similar patterns, having maximum values at 1500 rpm at 1/4 load and decreasing toward 2500 rpm at full load. B(a)P content was mainly dependent on the speed; that is, it increased as the speed became lower. In the differential scanning calorimetry of the particulates, two peaks were observed, one small

224 peak around 2 5 0 - 4 0 0 ° C and the other large peak around 5 0 0 - 7 0 0 ° C . T h e f o r m e r peak was attributed to f l a m m a b l e soluble fraction and the latter mainly to c a r b o n i z e d substances. The burnout t e m p e r a t u r e o f the particulates ranged f r o m 673 to 7 0 7 ° C , w h i c h was l o w e r as the carbon content b e c a m e l o w e r . T h e particulate matter is easier to burn as it is less c a r b o n i z e d and m o r e oxidized.

REFERENCES 1. Barth, D. S., and Blacker, S. M., J. Air Pollut. Contr. Assoc. 28:769 (1978). 2. US Federal Register, 45(45):14496 (1980). 3. Rondia, D., et al., Eds., Mobile Source Emissions Including Polycyclic Organic Species, Reidel, Dordrecht, 1983. 4. Japanese Environment Agency, White Paper on the Environment, 1986, pp. 177, 181.

AKIRA OBUCHI ET AL. 5. 6. 7. 8. 9. 10. 11. 12. 13.

14. 15. 16.

Plee, S. L., et al., SAE Technical Paper Series, No. 811195, 1981. Ludecke, O. A., and Dimick, D. L., Ibid., No. 830085, 1983. Obuchi, A., et al., J. Chromatogr. 288:187 (1984). Obuchi, A., et al., Ibid., 312:247 (1984). Obuchi, A., et al., Pollution Control (Kougai) 21(6):387 (1986). Jackson, J. L., Ind. Eng. Chem. 43(12):2869 (1951). Obuchi, A., et al., J. Jpn. Petrol Inst. 28(6):455 (1985). Zierock, K. H., et al., SAE Technical Paper Series, No. 830458, 1983. Ishiguro, T., and Suzuki, N., Society of Automotive Engineering of Japan, Preprint No. 831010, Tokyo, 1983. Donnet, J. B., and Voet, A., Carbon Black, Marcel Dekker, New York, 1976, p. 147. Smith, I. W., Fuel 57:409 (1978). Lewis, I. C., Carbon 20(6):519 (1982).

Received 25 August 1986; revised 14 May 1987