The performance and NOx (nitrous oxide) emissions of a biogas-fueled turbocharged internal combustion engine were investigated using one-dimensional cycle simulation. Analyses were carried out using the design of experiment method, and the results were verified by comparison with experimentally measured data. The combustion behaviors were improved as the CH4 content in the biogas increased. The brake power, brake thermal efficiency, and NOx emissions increased as the CH4 content or the boost pressure increased. Appropriate boost pressures to produce the same brake power at a given relative air/fuel ratio of 1.1 without boost were determined for each relative air/fuel ratio or each biogas composition considering brake power. In general, the lean operation limit was extended up to a relative air/fuel ratio of 1.5 with various biogas compositions and up to a relative air/fuel ratio of 1.7 for CH4:CO2 volume ratios of 65%:35% and 70%:30% without knocking. The maximum brake thermal efficiency was 35.9%, which was observed with a relative air/fuel ratio of 1.7 and a boost pressure of 1.44 bar at CH4:CO2 = 70%:30%. The NOx emissions were reduced by more than 90% via a reduction in the combustion temperature, which was achieved as a result of the lean combustion. (C) 2015 Elsevier Ltd. All rights reserved.