A method for optimizing operation of an engine driven by an electronic or digital system incorporating at least one parameter or one law or one mapping for engine injection, combustion or post-treatment comprising analyzing fuel composition from at least one sensor located in a fuel circuit of an engine comprising a filling system, a fuel tank, a pump, a fuel filter, an engine fuel system and a return circuit to the fuel tank, and selecting or modifying the parameter, law or mapping for injection, combustion or post-treatment according to a result of the analysis, wherein the fuel composition analysis comprises, spectroscopic analysis of the molecular structure of the hydrocarbons composing the fuel.