During combustion phenomena in Petrol engines, combustion is accompanied by flame, producing a very high rate of pressure rise resulted in detonation. Incidence of knock remains a significant barrier to achieving optimum performance potential. Experimental results are presented of the knocking behavior of a number of common gaseous fuels that include methane, hydrogen, propane and carbon monoxide and their mixtures. Comparison with the corresponding performance with liquid fuels is also made. Guidelines for achieving extended knock free operation with these fuels are to be outlined in this work. One of the major challenges of the automotive industry is to reduce the greenhouse gases, and especially CO2 emissions. Ethanol seems to have important assets to comply with these new constraints: it is extracted from the biomass and consequently has a good “well to tank” CO2 emission balance. Moreover, its properties, especially in terms of octane number and latent heat of vaporization allow a large improvement of the engine. The present book aims at taking stock of the ethanol path.