The fundamental effect of additives and the lean burn capability of methane combustion are investigated using cycle simulation and Latin hypercube sampling. The target engine is a spark-ignition engine fueled by methane and biogas mixed with hydrogen. The dominant variables were CO2 and H-2 content, spark timing, and excess air ratio (EAR). Varying the amount of additives (CO2 and H-2), spark timing, and EAR demonstrated that hydrogen plays an important role in extending the lean operating limit. The fractional factorial design of the experiment, Latin hypercube sampling, was applied to obtain the maximum brake torque (MBT) spark timing with variants in excess air ratios and additive content. When MBT spark timing is employed, the maximum mass fraction burned can be enhanced in the lean-burn region by increasing the hydrogen content with improved generating efficiencies under lean operating conditions. (C) 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.