Hybrids are fuels derived from combinations of different energy sources and which are generally formulated as solutions, emulsions, or slurries. The underlying objective of this program is to reduce the use of petroleum-derived fuels and/or to minimize the processing requirements of the finished hybrid fuels.Several hybrid fuel formulations have been developed and tested in a direct injection single-cylinder diesel engine. The formulations included solutions of ethanol and vegetable oils in diesel fuel, emulsions of methanol and of ethanol in diesel fuel; and slurries of starch, cellulose, and “carbon” in diesel fuel.Based on the progress to date, the solutions and emulsions appear to be viable diesel engine fuels if the economic factors are favorable and the storage and handling problems are not too severe. The slurries, on the other hand, are not to the same point of development as the solutions and emulsions. Although other solids do represent energy sources, the “carbons” appear to have the most potential for development into finished fuel slurries for current diesel engine designs.