3 Diesel-in-water-in-diesel emulsions
Double emulsions of the type oil/water/oil (O/W/O), where
oil is diesel, have been investigated by Lin and Wang [28,46–
49]. It was found that the viscosity of the double emulsions was
higher than for the normal emulsions but the higher viscosity
was not a problem in the use of the formulation as a fuel [46].
The benefits in terms of reduced emissions obtained with waterin-
diesel emulsions were obtained also with the O/W/O formulations.
The double emulsions seemed to give even lower levels
of NOx and CO than the regular emulsions although the differences
were relatively small. With regard to engine performance
and combustion characteristics the double emulsions gave a
higher exhaust gas temperature. Studies were made on the use
of diglyme (diethylene glycol dimethyl ether) as an additive
[47]. Diglyme is sometimes used to improve combustion characteristics
in diesel engines. It was found that addition of
diglyme caused stability problems for the double emulsions.
Most likely, the diglyme dissolves in the water phase and makes
this a better solvent for the surfactant, causing desorption of the
surfactant from the oil–water interface. Addition of diglyme
also caused stability problems for normal water-in-diesel emulsion
but the effect on stability was less pronounced in this case.