Energy & Fuels 2004, 18, 477-484
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Effects of a Combustion Improver on Diesel Engine Performance and Emission Characteristics When Using Three-Phase Emulsions as an Alternative Fuel Cherng-Yuan Lin* and Kuo-Hua Wang Department of Marine Engineering, National Taiwan Ocean University, Keelung 20224, Taiwan, Republic of China Received April 18, 2003
Diesel engines are widely used as the power source for inland and marine transportation, such as trucks, buses, ships, and agricultural and industrial vehicles. However, the pollutants of their emissionssespecially particulate matter (PM) and nitrogen oxides (NOx)spose a threat to human health as well as our environment. The use of emulsified diesel fuel as an alternative fuel for diesel engines has long been considered to be one of the feasible techniques for reducing pollutant emissions. However, most applications of emulsified fuel are limited to the two-phase water-inoil (W/O)-type emulsions. The three-phase emulsification technique for preparing oil-in-waterin-oil (O/W/O) or water-in-oil-in-water (W/O/W) is only applied in the medical, cosmetic, and food industries. This study explores the potential of using O/W/O emulsions with a diglyme additive, as an combustion improver, as a substitute for ASTM No. 2D diesel fuel. Engine performance and combustion characteristics were evaluated as the indicators for potential use. Experimental results showed that a diglyme additive enhances NOx emissions but deteriorates the emulsification stability of O/W/O three-phase emulsions. However, the addition of oxygenated diglyme to the O/W/O three-phase emulsions facilitated the combustion process, leading to higher combustion efficiency and lower fuel consumption rate and brake-specific consumption (bsfc). In addition, the smoke opacity of PM collected on the filter paper and CO emission decreased while fuel conversion efficiency, excess oxygen emission, and NOx emission increased with the addition of the oxygenated diglyme to both the two-phase W/O and three-phase O/W/O emulsions. The burning of the neat ASTM No. 2D diesel fuel produced the highest CO2, NOx, and O2 contents, as well as the highest smoke opacity from PM, fuel conversion efficiency, and exhaust gas temperature, but also produced the lowest CO emission and bsfc value.
Introduction The diesel engine is the most combustion-efficient engine; it has high thermal efficiency, a high power/ weight ratio, high fuel economy, and a simple and strong structural design. Diesel engines have been the main power source for on-land and on-sea transport vehicles such as buses, trucks, ships, and agricultural vehicles. It is expected that diesel engines will continue to be widely used in the foreseeable future. However, the emission of pollutants (in gaseous, liquid, and solid states) generated by the operation of a diesel engine jeopardizes the ecology of our planet and is detrimental to human health.1 Emissions from the operation of diesel engines contribute to the acceleration of the greenhouse effect, produce acid rains, destroy the ozone layer in the stratosphere, and affect the human respiratory system. Furthermore, vehicles powered by a diesel engine are considered one of the primary sources of air pollution, especially in metropolitan areas. Nitrogen oxides (NOx) and particulate matter (PM) are the two * Author to whom correspondence should be addressed. E-mail:
[email protected]. (1) Lin, C. Y. Reduction of Particulate Matter and Gaseous Emission from Marine Diesel Engines Using a Catalyzed Particulate Filter. Ocean Eng. 2002, 29 (11), 1327-1341.
major pollutants in diesel engine emissions. Photochemical smog produced from the conversion reaction of NOx and hydrocarbon (HC), in combination with ultraviolet (UV) sunlight, poses even greater damage to the environment. Moreover, PM, if it is inhaled and then stagnates in the respiratory system, can cause lung and bronchial diseases.2 The application of an emulsification technique to prepare the fuel has been considered to be one of the possible approaches to reduce the production of diesel engine pollutants, as well as the rate of fuel consumption. Microscopic water droplets in the dispersed phase are spread in the oil layer of the water-in-oil (W/O) twophase emulsion. The boiling point of water is lower than that of diesel fuel; therefore, microscopic water droplets are vaporized and explode outward through their enveloping oil layer to generate the microexplosion phenomenon. When this so-called “second atomization” occurs, the extent of the combustion reaction is improved. The commonly applied emulsification technique for diesel fuel or gasoline is mostly limited to the W/O two-phase emulsification. A mechanical or electrical (2) Carel, R. S. Health of Air Pollution. In Handbook of Air Pollution from Internal Combustion Engines; Sher, E., Ed.; Academic Press, Ltd.: London, 1998; pp 42-64.
10.1021/ef0300848 CCC: $27.50 © 2004 American Chemical Society Published on Web 01/28/2004
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Lin and Wang Table 1. Specifications of Surfactants Span 80 and Tween 80a
Figure 1. Physical structures of three-phase O/W/O emulsions.
agitating force is generally used to distribute microscopic water droplets uniformly into the other mutually immiscible liquid of the continuous diesel-oil phase. A suitable type of emulsifier is generally added to the emulsion to extend the separation time of the dispersal phase and the continuous phase, to improve the stability of the emulsion.3 Until recently, three-phase emulsions were only applied in the food and cosmetic industry, and also in the pharmacological industries, where it is used to control or prolong the release rate of the enveloped medicine. There are three common techniques for preparing threephase emulsions: phase inversion, mechanical agitation, and two-stage emulsification. Two-stage emulsification is, by far, the most commonly used among these three techniques. Three-phase emulsions are basically divided into two types: oil-in-water-in-oil (O/W/O) and water-in-oil-in-water (W/O/W) emulsions. These two types of three-phase emulsion structures can be applied in various ways, for example, using the solute concentration difference between the inner and outer solutions to generate an osmotic pressure to activate permeation through the inner and outer phases for the purpose of release or extraction.4,5 Miyamoto and co-workers6,7 found that the addition of certain suitable oxygenated agents could reduce the emission of smoke, PM, total hydrocarbon (THC), and NOx, as well as improve the thermal efficiency of diesel engines. They also determined that the noise generated during the combustion process could be reduced by enhancing the combustion characteristics and that the oxygen content of an oxygenated additive was the main factor for improving the thermal efficiency and for reducing the emission of pollutants. To take advantage of the three-phase emulsion structure, an oxygenated additive was used to mix the inner or outer phase of the emulsions. We used the oxygenated agent diglyme (diethylene glyco dimethyl ether) in this study. Figure 1 shows the physical structure of O/W/O emulsions with the oxygenated additive diglyme, in which the dispersed water phase separates both the inner oil phase and the outer oil phase. Diglyme is a colorless liquid with the (3) Zata, J. L.; Cueman, G. H. Assessment of Stability in Water-inOil-in-Water Multiple Emulsions. J. Soc. Cosmet. Chem. 1988, 39, 211. (4) Laugel, C.; Baillet, M.; Youenang, M. P.; Marty, J. P.; Ferrier, D. Oil-Water-Oil Multiple Emulsions for Prolonged Delivery of Hydrocortisone after Topical Application: Comparison with Simple Emulsions. Int. J. Pharm. 1998, 160, 109-117. (5) Garti, N. Double EmulsionssScope, Limitations and New Achievements. Colloids Surf., A 1997, 123, 233-246. (6) Miyamoto, N.; Ogawa, H.; Arima, T. Improvement of Diesel Combustion and Emissions with Addition of Various Oxygenated Agents to Diesel Fuels. SAE Tech. Pap. Ser. 1996, 962115. (7) Miyamoto, N.; Ogawa, H.; Nurun, N. M.; Obata, K.; Arima, T. Smokeless, Low NOx, High Thermal Efficiency, and Low Noise Diesel Combustion with Oxygenated Agents as Main Fuel. SAE Tech. Pap. Ser. 1998, 980506.
a
From ref 9.
chemical formula of CH3(OCH2CH2)2OCH3. The oxygen content, cetane number, and heating value of diglyme are 35.8 wt %, 126, and 24.5 MJ/kg, respectively.8 The effects on engine performance and emission characteristics of diesel engines when using diglyme as an oxygenated additive for diesel fuels, W/O emulsions, and O/W/O emulsions are studied in this paper. Experimental Details Because diglyme has dipolar emulsification characteristics, it is miscible with diesel oil and water. Moreover, the oxygen content of diglyme is as high as 35.8 wt %. Hence, diglyme can be used as an oxygenated agent in diesel fuels and in O/W and W/O two-phase emulsions, as well as O/W/O three-phase emulsions. The volumetric ratio of diglyme addition to the twophase or three-phase diesel-oil emulsions is expressed (as a percentage) as diglyme/(diesel oil + distilled water) × 100. In this study, diglyme was mixed with either diesel fuel or water, using an electromagnetic heating stirring apparatus prior to the emulsion preparation. The emulsifier addition was 2% of the combined volume of diesel oil and distilled water in the two- or three-phase emulsion, and the emulsifier addition mixture for preparing the O/W/O three-phase emulsions was set at a hydrophilic-lipophilic balance (HLB) of 8. This intent of this experiment is to explore the effects of an oxygenated addition to two- and three-phase emulsions on diesel engine performance and emission characteristics. Two surfactantssSpan 80 (lipophilic, HLB ) 4.3) and Tween 80 (hydrophilic, HLB ) 15)swere used in this experiment to prepare the three-phase O/W/O emulsions. Table 1 shows the specifications of the surfactants Span 80 and Tween 80.9 ASTM No. 2D diesel fuel, a product of the Chinese Petroleum Co. in Taiwan, was used as the emulsion oil phase, and its specifications are listed in Table 2. The two-stage mechanical agitating emulsification method was used to prepare the O/W/O three-phase emulsions, which contained 10% water and an oxygenated diglyme. The surfactant mixture with HLB ) 8 was also added to reduce the interfacial tension and to enhance the affinity force between the diesel fuel and water phases. A homogenizing and emulsifying machine, set at a stirring speed of 5000 rpm, was used to mix the diesel fuel and water phases. Diglyme was also added to both the W/O two-phase emulsions with 10% water content and the neat ASTM No. 2D diesel fuel for comparison purposes. A four-stroke, natural-air-aspirated, water-cooled, (8) Arthur, E. R. The Condensed Chemical Dictionary, 7th ed.; Nostrand Reinhold Co.: New York, 1973; p 387. (9) Surfactants User’s Manual; Kao Surfactant Co., Japan, 1991.
Diesel Performance with W/O/W and O/W/O Emulsions
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Figure 2. Schematic diagram of the experimental setup. Legend is as follows: 1, vacuum pump; 2, air flow meter; 3, cyclone particle collector; 4, exhaust gas cooler; 5, homogenizing machine; 6, diesel fuel tank; 7, diesel fuel filter; 8, fuel flow meter; 9, exhaust manifold; 10, computerized data acquisition system; 11, eddy current dynamometer; 12, personal computer for analyzing engine test data; and 13, body of the diesel engine. Table 2. Specifications of the ASTM No. 2D Diesel Fuela item
specification
manufacturer kinematic viscosity at 40 °C carbon content hydrogen content sulfur content density at 15 °C flash point heating value cetane index distillation temp. IBP 10 vol % 20 vol % 50 vol % 90 vol % 95 vol % end point recovery residue
Chinese Petroleum Co., Taiwan 2.479 cst 86.13 wt % 13.93 wt % 0.34 wt % 0.8324 kg/L 70 °C 10105 cal/g 50.9
a
171.1 °C 215.3 °C 227.6 °C 257 °C 316.7 °C 338.4 °C 351.7 °C 97.9 vol % 1.6 vol %
Test report from Chinese Petroleum Company, Taiwan, ROC.
direct injection diesel engine with a displacement volume of 3856 cm3 and an eddy current dynamometer was used for engine performance and emission characteristics measurements. A computerized automatic engine test and data acquisition system was used to acquire the experimental data. The schematic diagram of the experimental setup is illustrated in
Figure 2. A gas analyzer was used to measure the diesel engine emission (e.g., CO, NOx, and SO2) and the combustion efficiency. The exhaust gas temperature of the test engine was measured by a K-type thermocouple. A cyclone dust collector with fiberglass paper was used to collect the PM
