Potential and knocking characteristics of a hydrogen-natural gas blend (HCNG) engine with a high compression ratio were examined from a commercial viewpoint since lean combustion with HCNG under a wide-open throttle (WOT) condition requires a high-charging-capacity turbocharger. Supercharging of intake air to extend the lean limit was investigated for a turbocharged, heavy-duty natural gas-fueled engine. Effects of compression ratio changes on fuel economy were assessed in terms of thermal efficiency and torque characteristics. Extension of the lean limit to an excess air ratio of 1.8 for an HCNG engine under WOT conditions is realizable using a supplementary supercharging system. Thermal efficiency improvement at high compression ratios is reduced under relatively rich mixture conditions because spark timing is retarded to avoid knocking. The excess air ratio corresponding to maximum thermal efficiency decreases to 1.6 for an HCNG engine due to the decrease in exhaust gas energy for intake-air charging. Copyright (C) 2012, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.