In this study, an experimental investigation on a naturally aspirated (NA), 8-L spark ignition engine fueled by biogas with various methane concentrations - which we called the N(2) dilution test - was performed in terms of its thermal efficiency, combustion characteristics and emissions. The engine was operated at a constant engine rotational speed of 1800 rpm under a 60 kW power output condition and simulated biogas was employed to realize a wide range of changes in heating value and gas composition. The N(2) dilution test results show that an increase of inert gas in biogas was beneficial to thermal efficiency enhancement and NO(x) emission reduction, while exacerbating THC emissions and cyclic variations. Then, as a way to achieve stable combustion for the lowest quality biogas, H(2) addition tests were carried out in various excess air ratios. H(2) fractions ranging from 5 to 30% were blended to the biogas and the effects of hydrogen addition on engine behavior were evaluated. The engine test results indicated that the addition of hydrogen improved in-cylinder combustion characteristics, extending lean operating limit as well as reducing THC emissions while elevating NO(x) generation. In terms of efficiency, however, a competition between enhanced combustion stability and increased cooling energy loss was observed with a rise in H(2) concentration, maximizing engine efficiency at 5-10% H(2) concentration. Moreover, based on the peak efficiency operating point, a set of optimum operating conditions for minimum emissions with the least amount of efficiency loss was suggested in terms of excess air ratio, spark ignition timing, and hydrogen addition rate as one of the main results. Copyright (C) 2011, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.