The production of hydrogen is studied numerically under uncatalyzed partial oxidation and homogeneous charge compression ignition (HCCI) conditions in an internal combustion engine fueled by natural gas. The HCCI process is modeled by a single-zone variable volume reactor using a global heat transfer model and elementary-step reaction mechanisms. Numerical optimization is applied to maximize the hydrogen yield at the end of the expansion stroke by varying the equivalence ratio, engine speed and initial pressure for a fixed initial temperature. Suitable constraints were defined, including peak pressure and bounds to the optimization variables. From these results, maximum hydrogen yield profiles and the associated operating parameter profiles as functions of initial temperature were obtained. The profiles exhibit strong linear dependency with initial temperature. Reaction flow analysis was performed to gain detailed insight into the chemical processes involved when the engine is run under optimal conditions for a maximum hydrogen yield. The integral reaction flow analysis shows that substantial amounts of hydrogen are produced from precursors, which are also valuable products, such as methanol, formaldehyde and ethylene. Copyright (C) 2015, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.