A comprehensive analysis of the effect of ethanol, methane and methane-hydrogen blend on the combustion process in an optical PFI engine

The energy crisis and environmental issues make the alternative fuels, both liquid and gaseous, even more attractive because of their potentiality in reducing the fuel consumption and the pollutant emissions. Ethanol is the most promising alternative liquid fuel for spark ignition engine. It has higher octane number, resulting in good anti-knock characteristics as well as the possibility to work with higher compression ratios improving the engine efficiency. The higher heat of vaporization compared to gasoline leads increased power output. Moreover, the higher oxygen content allows a more complete combustion and the reduced emissions. Among gaseous fuels, methane is considered one of the most interesting. It has wider flammable limits and better anti-knock properties. Moreover, it is characterized by lower CO2 emissions. On the other hand, the slow flame propagation speed and its poor lean-burn capability result in lower engine power output. The addition of a high burning velocity fuel, such as the hydrogen, contribute to the improvement of the combustion process in terms of burning velocity and the extension of lean operation limit. The objective of this paper is the analysis of the effect of different fuels on the engine performance and emissions. Experimental investigations were carried out in an optically accessible small single-cylinder, spark ignition four-stroke engine. It was equipped with the cylinder head of a Port Fuel Injection (PFI) commercial 250 cc engine. The engine was fuelled with gasoline, ethanol, methane and a blend of hydrogen in methane. Optical measurements were performed to analyse the combustion process with high spatial and temporal resolution. In particular, the optical techniques based on 2D-digital imaging were used to follow the flame propagation in the combustion chamber. UV-visible spectroscopy allows detecting the chemical markers of combustion process such as the radicals OH* and CH*. The exhaust emissions were characterized by means of gaseous analysers. The measurements were performed under steady state conditions at different engine load.