TECHNOLOGY FOCUS - ORGANIC CHEMISTRY - Preferred Compounds: The di- or polyhydroxy hydrocarbon derivative contains: (i) a di- or polyhydroxy 2-6C hydrocarbon structure, preferably from (di)-ethylene glycol, pentaerytritol, or diglycerol; or (ii) a sugar structure, preferably from glucose, fructose, sorbitol, or isosorbide. The di- or polyhydroxy hydrocarbon derivative is at least partially esterified with a straight or branched, 1-5 alkyl carboxylic acid (preferably formic acid, acetic acid, or propionic acid) and/or at least partially etherified with a straight or branched 1-5C alkyl ether (preferably tert-butyl or tert-amyl ether). The OH-functionalities of di- or polyhydroxy hydrocarbon derivative are completely derivatized by the ether moieties and/or ester moieties. The ratio of ester to ether functionalities in the derivative is larger than 1. The derivative is triacetin. Or the ratio of ester to ether functionalities in the derivative is smaller than 1. The OH-functionalities of the di- or polyhydroxy hydrocarbon derivative are partially derivatized by the ester moieties and/or ether moieties, and the motor fuel composition is a diesel fuel composition, or a gasoline, a jet fuel, or a gasoline/ethanol mixture. The derivative is diacetin, monoacetyl isosorbide, diacetyl isosorbide, and/or glyceryl tert-butyl ether. At least partially esterified and/or at least partially etherified, di- or polyhydroxy, 2-12C hydrocarbon derivative is further used as an octane number improving agent. The di- or polyhydroxy 2-12C hydrocarbon derivative is selected from ethylene glycol, diethylene glycol, glycerol, diglycerol, pentaerythritol, isosorbide, a sugar comprising a tetrose, pentose, hexose (preferably glucose, fructose, xylose, sorbitol, or mannitol). Preferred Composition: The hydrocarbon derivative is present in a motor fuel composition in an amount of 1-10 (preferably 1-5, especially 1.5-3) vol./vol.%. The motor fuel composition is a diesel fuel composition, optionally comprising a biodiesel composition, boiling at 160-370 degrees C. The ethanol phase of the gasoline/ethanol mixture contains 1-7 (preferably an azeotropic mixture with water containing 4-5) vol.% of water. Preferred Method: In the method (M1): the 1-5C alkyl ester of a 1-5C carboxylic acid is selected from 1-5C alkyl acetate (preferably methyl acetate or ethyl acetate); and the reaction is executed with a molar excess of 1-12 mol 3-5C alkyl ester of 1-5C carboxylic acid to 1 mol 10-30C fatty acid triglyceride (preferably 6-12 mol 1-5C alkyl ester of 1-5C carboxylic acid, especially methyl acetate), to 1 mol 10-30C fatty acid triglyceride. The catalyst is selected from a basic catalyst (preferably potassium methanolate), a solid or liquid acid catalyst (preferably sulfuric acid), a heterogeneous catalyst; an acid ion exchanger (preferably Amberlyst-15); or an enzymatic catalyst. The solid acid catalyst is selected from a zeolite (preferably H-ZSM-5, Y and Beta), and a mixed metal oxide (preferably sulfated zirconium oxide). Starting from a fatty acid triglyceride, of formula (I) is transesterified with a lower alkanol, R4OH, in the presence of a catalyst (preferably a basic catalyst) to obtain a 2-phasic mixture, consisting of a glycerol containing phase, and a R4 alkyl esters of 10-30 fatty acids containing phase; the two phases are dried, and separated, if desired; and the glycerol containing phase is reacted with a R5 alkyl ester of a R6 alkyl carboxylic acid, to obtain the agent as a mixture of a glyceryl alkyl esters containing fraction and a fatty acid alkyl esters containing fraction, which can be purified, if desired. The etherification or esterification reactions are driven by reactive distillation. The obtained phases are dried by azeotropic distillation. The fatty acid triglyceride is derived from a pure vegetable oil, such as rape seed oil, sunflower oil, palm oil, or soy oil.R4=1-5C alkyl (preferably methanol);R5 and R6=1-5C alkyl (preferably 1-5C alkyl ester of 1-5 carboxylic acid, especially methyl acetate or ethyl acetate). EXAMPLE - To a standard fossil diesel fuel composition, a mixture of an etherified (tert-butyl ether) and esterified (acetate) glycerol derivative in a concentration of 2 (vol./vol.%) was added. The ether to ester ratio was about 1, and the functionalization of the OH groups was virtually complete. The fuel composition was evaluated for fuel saving in a test run executed with a standard diesel engine. The fuel savings were measured at an electronically controlled constant load in a test run in the stationary phase. It was demonstrated that in the test run about 6% fuel saving, accompanied by about 25% soot reduction, compared to the standard diesel fuel without the additives, was observed.