Internal combustion engine development focuses mainly on two aspects: fuel economy improvement and pollutant emissions reduction. As a consequence, light duty spark ignition (SI) engines have become smaller, supercharged, and equipped with direct injection and advanced valve train control systems. The use of alternative fuels, such as natural gas (NG) and liquefied petroleum gas (LPG), thanks to their lower cost and environmental impact, widely spread in the automotive market, above all in bifuel vehicles, whose spark ignited engines may run either with gasoline or with gaseous fuel. The authors in previous works experimentally tested the strong engine efficiency increment and pollutant emissions reduction attainable by the simultaneous combustion of gasoline and gaseous fuel (NG or LPG). The increased knock resistance, obtained by the addition of gaseous fuel to gasoline, allowed the engine to run with stoichiometric mixture and best spark timing even at full load. In the present work, the authors extended the research by testing the combustion of gasoline–NG mixtures, in different proportions, in supercharged conditions, with several boost pressure levels, in order to evaluate the benefits in terms of engine performance, efficiency, and pollutant emissions with respect to pure gasoline and pure NG operation. The results indicate that a fuel mixture with a NG mass percentage of 40% allows to maximize engine performance by adopting the highest boost pressure (1.6 bar), while the best efficiency would be obtained with moderate boosting (1.2 bar) and NG content between 40% and 60% in mass.

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