This work was aimed at improving the performance and extending the load range of hydrogen fueled homogeneous charge compression ignition (HCCI) engine through charge temperature regulation and addition of carbon dioxide in order to control the combustion phasing. Intake charge temperature and equivalence ratio were varied from 130 degrees C to 80 degrees C and 0.19 to 0.3 respectively. In the neat hydrogen mode it was possible to operate the engine only until a brake mean effective pressure (BMEP) of 2.2 bar. Higher charge temperatures lead to knocking and advanced combustion. At any equivalence ratio the lowest possible charge temperature is the one that leads to the highest thermal efficiency. Addition of carbon dioxide retarded the combustion process and improved the thermal efficiency and also extended the load range to a BMEP of 3.1 bar. Efficiencies of hydrogen HCCI mode were higher than the conventional diesel mode with negligible level of NO emissions. Copyright (C) 2014, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.