A method for operating an internal combustion engine utilizing a fuel mixture containing a suspension in air at a concentration of about 200 mg of fuel per liter of air, of a solid fuel including at least one of a cereal flour, a cotton flour, a soybean flour, a potato flour, a cassava flour, a dehydrated chocolate powder and a dehydrated milk powder, the solid fuel being in the form of a powder having an average particle diameter and median particle diameter at least 150 .mu.m.