This paper presents further results of the study on basic combustiQn characteristics of gaseous fuels simulated for a bio − gas produced through a biomass gasi 丘cation process with a catalyzer ． CO2 dilution effect was examined for the individual fuel component of CH4 ， CO and II2 in a single − cylinder spark − igni ' tion engine under thb condition of WOT ， MBT and a constant speed of 1500 rpm ． For these three kinds of fuels， until a cert 乱 in level of CO2 dilution ， the thermal eMciency does not vary ，