Performance and combustion analysis of a supercharged double-fuel spark ignition engine

In order to increase fuel economy and reduce pollutant emissions in the last decades light duty spark ignition (SI) engines have become smaller, supercharged and equipped with direct injection. A suitable alternative to oil derived fuel is represented by gaseous fuels, such as Natural Gas (NG) and Liquefied Petroleum Gas (LPG), whose higher knock resistance and better mixing capabilities substantially improve vehicle fuel economy and pollutant emissions. The simultaneous combustion of gasoline and gaseous fuel (Double-Fuel operation, DF) in a naturally aspirated SI engine has already been investigated in the past also by the same authors, proving remarkable results in terms of engine efficiency increment and exhaust emissions reduction. In this paper the authors present the results of a new methodical experimental study aimed to investigate engine performance, efficiency and pollutant emissions obtained on a supercharged SI engine operated in double fuel mode, with comparison to the use of "reference" pure fuels (i.e. gasoline and NG). A detailed heat release analysis is also performed with the aim to highlight the effect of fuel mixture composition (i.e. the proportion between gasoline and NG) and of charging pressure on the combustion speed.