Fuels produced using microemulsification technology are called “hybrid Fuels”. They are called ionic or non-ionic depending on the type of surfactant present. For example, those hybrid fuels containing a basic nitrogen compound are termed ionic while those consisting only of plant oil, aqueous ethanol, and another alcohol, such as 1-butanol, are termed non-ionic or detergentless microemulsions, referring to the absence of a surfactant. Mixtures of hexadecane, 1-butanol, and 95% ethanol are examples of detergentless microemulsions. C1 to C3 alcohols (methanol, ethanol, propanol) are used as viscosity-lowering additives while longer chain alcohols and alkylamines serve as surfactants. For example, n-butanol is claimed to produce microemulsions that are more stable and have lower viscosity than those made with methanol or ethanol. Surfactants are important constituents in the production of microemulsified hybrid fuels. They increase the solubility of diesel-plant oil-water blends producing single phase, homogeneous mixtures.
Various formulations of hybrid fuels have been tested in engines using the 200-hr Engine Manufacturers Association (EMA) test. A microemulsion fuel containing soybean oil, methanol, 2-octanol, and a cetane enhancer was the cheapest vegetable oil fuel ever to pass the EMA test. Goering found that eight parts of soybean oil, when emulsified with two parts ethanol and five parts of 1-butanol as stabilizer, performed as well as diesel fuel and was able to start a cold engine. Two studies of palm oil, diesel fuel and 5-10% water microemulsions revealed performance comparable to diesel fuel with less engine wear. Sapaun et al reported that studies in Malaysia with palm oil as diesel fuel substitutes exhibited encouraging results. Performance tests indicated that power outputs were nearly the same for 100% palm oil, blends of palm oil and diesel fuel, and 100% diesel fuel. Short-term tests using palm oil fuels showed no signs of adverse combustion chamber wear, increase in carbon deposits, or lubricating oil contamination.
Using the same EMA 200-hour engine screening test, several fuels showed technical promise with performance comparable to that of diesel fuel. These include the following blends: (a) diesel:soybean oil:butanol:cetane improver (33:33:33:1); (b) diesel:soybean oil:190 proof ethanol:butanol, (50:25:5:20); and (c) soybean oil:methanol:2-octanol:cetane improver, (53:13:33:1). The fuel injection characteristics were also studied and recorded using a pressure vessel, fuel injection system, and high speed motion picture camera, in a quiescent nitrogen atmosphere at 4800C and a pressure of 4.1MPa. It was observed that the injection and atomization characteristics were markedly different from those of diesel fuel. Heating the vegetable oils to lower their viscosities increased spray penetration rate, reduced spray cone angles, and resulted in spray characteristics that more closely resembled those of diesel fuel. Significant chemical changes occurred following injection such that appreciable amounts of C4-C16 hydrocarbons and free carboxyl groups were detected at about 400 microseconds after injection.
Unsaturated fatty acids are used to produce microemulsions of alcohols with diesel fuel since saturated fatty acids produce unsatisfactory results because of the formation of crystalline phases upon refrigeration. The addition of n,ndimethylamino ethanol (DMAE) produces microemulsions with satisfactory viscosity. Two types of fuels were tested, one ionic and another non-ionic: (a) 66.7% diesel fuel, 16.7% ethanol (95%), 12.5% soybean acids, and 4.1% DMAE (ionic); and (b) 66.7% diesel fuel, 11.1% ethanol (95%), and 22.2% 1-butanol (non-ionic)13. Both hybrid fuels gave acceptable performance with improved brake thermal efficiency and lower exhaust temperatures. Smoke and CO levels were lower but the unburned hydrocarbon levels were higher compared to pure diesel fuel. The non-ionic microemulsion was superior to the ionic one in those properties relevant to good engine performance but the ionic hybrid fuel showed better water tolerance. A hybrid fuel consisting of 50% diesel fuel, 25% degummed, alkalirefined soybean oil, 5% aqueous ethanol (95%) and 20% 1-butanol was studied by the same group of researchers using the 200-hr EMA (Engine Manufacturers Association) test. The engine running on this hybrid fuel completed the EMA test without difficulty, causing less engine wear than diesel fuel but producing greater amounts of carbon and lacquer on the injector tips, intake valves and tops of the cylinder liners. The engine performance declined by 5% at the end of the test.
Microemulsified hybrid fuels using plant oils but without diesel fuel have also been widely studied. A hybrid fuel comprising a plant oil, a lower (C1-C3) alcohol, water, and a surfactant system consisting of a trialkylamine or the reaction product of a trialkylamine with a long-chain fatty compound was reported. In another patent, a hybrid fuel was produced using a plant oil, a C1-C3 alcohol, water, and 1-butanol as nonionic surfactant. This formulation produced a hybrid fuel with acceptable viscosity and compared favorably to diesel fuel in terms of engine performance. Another hybrid fuel formulation consisted of a plant oil, methanol or ethanol, a straight-chain isomer of octanol, and water, which again exhibited high water tolerance, acceptable viscosity and performance comparable to diesel fuel. Another patent reported the formulation of a hybrid fuel from degummed rapeseed oil, water, and a surfactant such as an alkaline soap or a potassium salt of fatty acids. Another reported hybrid fuel formulation consisted of fatty esters, aqueous alcohol, and small amount of alkali metal soap18. All these microemulsified hybrid fuels showed acceptable performance comparable to that of diesel fuel.
Two other hybrid fuels were tested. One was non-ionic consisting of 53.3% soybean oil, 13.3% aqueous ethanol (95%) and 33.4% 1-butanol and the other was ionic composed of 52.3% soybean oil, 17.4% aqueous ethanol (95%), 20.5% 1-butanol, 6.54% linoleic acid and 3.27% triethylamine. The two hybrid fuels performed nearly as well as diesel fuel despite their lower cetane numbers and less energy content. The non-ionic hybrid fuel produced nearly as much engine power as pure diesel fuel. The higher viscosity of the hybrid fuels produced a 16% increase in the mass of each fuel injection at maximum power, but the injections contained 6% less energy than those of diesel fuel. However, there was a 6% gain in thermal efficiency. In contrast, in a separate study, a non-ionic microemulsified hybrid fuel comprising of alkali-refined, winterized sunflower oil (53.3%), 95% aqueous ethanol (13.3%) and 1-butanol (33.4%) encountered major problems of incomplete combustion at low-load engine operation. In addition, carbon residues were deposited on the piston ring grooves and in the intake ports, and premature injection nozzle deterioration was experienced.