Microemulsified fuels

Microemulsions form spontaneously and exhibit nano-disperse structures. In contrast to emulsions there is no additional energy input necessary for the production of a microemulsion. The formation is thermodynamically favoured due to the ultra-low interfacial tension between the oil and water domains. The microemulsified fuels are in principle thermodynamically stable for an unlimited period of time only the chemical stability of the single components could be a limiting factor. A further advantage of microemulsions in contrast to emulsions is the fact that the water content can be adjusted over a broad range. Therefore, the combustion process can be customised to specific needs. An important criterion for a microemulsion to be used as fuel is that the one-phase region extends over a wide temperature range (Fig. 11.4). Mixtures of ionic and non-ionic surfactants, which exhibit almost temperature-invariant phase behaviour by optimal composition, are suitable to meet these standards. 

Figure 11.5 Soot in units of the filter smoke number (FSN) (a) and NOx (b) emissions of microemulsified fuels with varying water content for combustion measured as function of torque M. The measurements are carried out on a Deutz engine, 82 kW, 4-cylinder, without EGR at constant 1500 rpm as a function of the engine load condition (Institute for Mechanical Engineering and Vehicle Technology, University of Applied Science, Trier).

Fernandes, M. R. 2005. Development of a new diesel-based microemulsified fuel, M.Sc. dissertation, PPGEQ, Chemical Engineering Department, UFRN, Brazil (in Portuguese). 

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. 

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