Performances and Emissions Improvement of an S.I. Engine Fuelled by LPG/Gasoline Mixtures

As is known gaseous fuels, such as Liquefied Petroleum Gas (LPG) and Natural Gas (NG), thanks to their good mixing capabilities, allow complete and cleaner combustion than normal gasoline, resulting in lower pollutant emissions and particulate matter. Some of the automobile producers already put on the market “bi-fuel” engines, which may be fed either with standard gasoline or with LPG. These engines, endowed of two separate injection systems, are originally designed for gasoline operation; hence they do not fully exploit the good qualities of LPG, such as its better knocking resistance, which would allow higher compression ratios. Moreover, when running with gasoline at medium high loads, the engine is often operated with rich mixture and low spark advance (with respect to the maximum brake torque value) in order to prevent from dangerous knocking phenomena: this produces both high hydrocarbon and carbon monoxide emissions and high fuel consumption. Starting from these observations, the authors experimentally investigated on the simultaneous combustion of LPG- gasoline mixtures in stoichiometric proportion with air (with different LPG/gasoline mass ratios), so as to exploit the good qualities of both fuels to obtain cleaner and more efficient combustions: the addition of LPG to the gasoline-air mixture in fact raises knocking resistance, allowing thus to run the engine with both “overall stoichiometric” mixture and more efficient spark advance even at full load, while the stoichiometric A/F ratio allows to minimize pollutant emissions. In this paper the authors present the results of an extensive experimental study in terms of engine efficiency increments and reduction of pollutant emissions with respect to the pure gasoline operation.