Biogas is a promising alternative fuel to reduce the consumption of petroleum-based fuels in internal combustion (IC) engines. In this work, the effect of various biogas compositions on the performance, combustion, and emission characteristics of a spark-ignition (SI) engine is investigated. Additionally, the effect of Wobbe index (WI) of various fuel compositions was also evaluated on the operational limits of the engine. While considering a wide range of biogas compositions (including bio-methane), the percentage of carbon dioxide (CO2) (in a blend of methane and CO2) was increased from 0 to 50% (by volume). A single-cylinder, water-cooled, SI engine was operated at 1500 rpm over a wide range of operating loads with compression ratio of 8.5:1. With the increase in WI of the fuel, both low (limited by coefficient of variation (COV) of indicated mean effective pressure (IMEP)) and high (limited by pre-ignition) operating loads were decreased; however, it was found that the overall operating range was increased. Results also showed that for a given operating load, with the increase of CO2 percentage in the fuel, the brake thermal efficiency was decreased, and the flame initiation and combustion durations were increased. The brake thermal efficiency was decreased from 16.8% to 13.7%, when CO2 was increased from 0% to 40% in methane-CO2 mixture at 8 N.m load. Concerning to emissions, a considerable decrease was noted in nitric oxide, whereas hydrocarbon, carbon monoxide and carbon dioxide emissions were increased, with the increase in CO2 percentage. Copyright © 2019 by ASME