The results of previous experimental researches showed that great advantages can be achieved, both in terms of fuel consumption and pollutant emissions, in bifuel vehicles by means of the double-fuel combustion, i.e., the simultaneous combustion of gasoline and a gaseous fuel, such as liquefied petroleum gas (LPG) or natural gas (NG). The substantial increase in knock resistance pursued by adding LPG to gasoline, which allowed to maintain an overall stoichiometric proportion with air also at full load, is not documented in the scientific literature and induced the authors to perform a proper experimental campaign. The motor octane number (MON) of LPG–gasoline mixtures has been hence determined on a standard cooperative fuel research (CFR) engine, equipped with a double-fuel injection system in order to realize different proportions between the two fuels and electronically control the overall air–fuels mixture. The results of the measurement show a quadratic dependence of the MON of the mixture as function of the LPG concentration evaluated on a mass basis, with higher increase for the lower LPG content. A good linear relation, instead, has been determined on the basis of the evaluated LPG molar fraction. The simultaneous combustion of LPG and gasoline may become a third operative mode of bifuel vehicles, allowing to optimize fuel economy, performances, and pollutant emissions; turbocharged bifuel engines could strongly take advantage of the knock resistance of the fuels mixture thus adopting high compression ratio (CR) both in pure gas and double-fuel mode, hence maximizing performance and reducing engine size. The two correlations determined in this work, hence, can be useful for the design of future bifuel engines running with knock safe simultaneous combustion of LPG and gasoline.

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