Diesel fuels microemulsions

Soybean oil is converted into soy FAME through transesterification with methanol. This product is an excellent substitute for diesel fuel with no engine adjustment required and no loss in efficiency (Knothe Dunn, 2005). Besides transesterification to methyl esters, other approaches also were explored for utilizing soybean oil as fuel. These are diluted with conventional petroleum diesel fuel, microemulsions (co-sol-vent blending), and pyrolysis (Schwab et al., 1987). A detailed discussion on biodiesel is provided in the Chapter Bioenergy and Biofuels from Soybeans, and also elsewhere (Knothe Dunn, 2005 Reaney et al., 2004). 

This technology has been applied to conventional naphtha- and distillate-based fuels. Studies completed by the United States Army in the 1970s and 1980s confirmed that diesel-water microemulsions could be an effective fire resistant fuel. It was found that water purity or hardness of 50 ppm or less was required to provide an effective microemulsion. 

A microemulsion fuel suitable for use in diesel engines has been prepared from diesel fuel, ethanol, traces of water and cationic surfactants as emulsifiers, plus other additives [94]. Suitable cationic surfactants are alkyl polyamines and their alkoxylates. The fuels benefit from improved lubricity. 

However, if we also take the smoke behavior into account, the advantage of using a microemulsion fuel is clearly evidenced (Figures 6a and 6b). The microemulsion fuel produces a simultaneous decrease in smoke and NO. This is a striking difference from the effects for an ordinary diesel fuel in which the change in injection timing causes a decrease in NO but, at the same time, an immense increase in the soot. 

Another possible application where the typically low viscosity of microemulsions is very useful is in the preparation of alternative fuels for diesel engines. Such hybrid fuel microemulsions containing vegetable oil [13] and alcohols, with 1-butanol [14] or a lower trialkylamine [15] acting as surfactant attracted interest some years ago, and such systems were tested for their practical application [16]. Similar systems containing triglyceride, aqueous ethanol, and 1-butanol [17] or long-chain fatty alcohols [18] were also studied for the same purpose. 

Frequently, phase separation effected upon incorporation of additives or cosolvents is a major concern when developing novel fuel formulations. Microemulsion-based mixtures can overcome this problem, and has been the focus of more recent works. In that respect, Friberg and Force, in 1976, patented a diesel-based microemulsion formulation that could be used as fuel, with reduced NO emissious when compared to pure diesel [52]. Subsequent works have focused on phase behavior, stability, and performance of different mixtures, most of which involving surfactant-based mixtures [26,39], but it is worth considering the recent advances in the use of microemulsified systems incorporating other fluids like vegetable oils and alcohols. 

The engine used was a one-cylinder direct-injection test engine at Saab-Scania, Sodertalje, Sweden. During the tests, maximum pressure, fuel consumption, exhaust temperature, CO, CO2, NO, NO2, O2, HC, and smoke were registered at varied injection timings, loads, and speeds. The performance of three microemulsion fuels was compared with a reference fuel of pure diesel oil with the cetane number of 43. The data of the microemulsion fuels are given in Table II. 

Figure 5. Relation between the relative NO emission and fuel consumption at various injection timings 28, 23, 18, and O 13 ""BTDC, The reference fuel ( ) is a diesel oil with cetane number 43. Fuel 1 (mmti), fuel 2 (irkif ) and fuel 3 (---) are microemulsion fuels with 10, 20, arid 30% water respec-...

By careful choice of emulsifiers it is possible to provide microemulsions based on diesel oil which exhibit an ignition performance suitable for high-speed diesel engines. In contrast to emulsion fuels examined previously (23), these microemulsions show a pronounced net benefit in the NO and smoke emissions. The amounts of unbumed hydrocarbon and CO in the exhausts do increase but may be reduced by a catalytic afterburner. 

Changing fuel composition by adding surfactants and water decreases its specific energy, which has to be considered in view of effective specific consumption. The estimation of the lower heat value after W. Boie [32] leads to the values listed in Table 11.1 for the examined microemulsions. As can be seen from the numbers the amount of fuel being energy equivalent to diesel is reduced with increasing water content during the combustion of microemulsions. 

Goering, C.E. R.M. Campion A.W. Schwab E.H. Pryde. Evaluation of soybean oil-aqueous ethanol microemulsions for diesel engines. Proceedings of the international conference on plant and vegetable oils as fuels, ASAE Publ. 4-82 Veg. Oil Fuels) 1982, 279—286. 

Silva, E.J., Zaniquelli, M.E.D., and Loh, W. 2007 Light-scattering investigation on microemulsion formation in mixtures of diesel oil (or hydrocarbons) + ethanol + additives. Energy Fuels 21 222-226. 

We have reported on mixtures comprising diesel and soy oil, water, and surfactants that provide an economy of 25% in fuel consumption compared to pure diesel [32], Incorporation of additives to ethanol-diesel mixtures can ultimately enhauce the solubility of the final product, featuring typical microemulsion properties hke thermodynamical stability and macroscopical transparency [53]. 

FIGURE 15.17 Specific fuel consumption as a function of percentage of maximal power developed by engines operated with commercial and microemulsion-based diesel. 

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