The wide range of hydrogen's flammable limits enables ultra-lean combustion. A lean burn reduces the combustion temperature, increases thermal efficiency, and reduces knock, which is a serious problem in a spark ignition (SI) engine. The anti-knock improvement from hydrogen addition makes it feasible to increase the compression ratio (CR) and further improve the thermal efficiency. Herein, the effects of the CR on performance and emission characteristics were investigated using an 11-L heavy-duty SI engine fuelled with HCNG30 (CNG 70 vol%, hydrogen 30 vol%) and CNG. These fuels were used to operate an engine with CRs of 10.5 and 11.5. The results showed that thermal efficiency improved with an increased CR, which significantly decreased CO2 emission. On the other hand, the NOx emission was largely increased. Nevertheless, for HCNG30, a CR of 11.5 improved thermal efficiency by 6.5% and decreased NOx emission by over 75%, as compared to a conventional CNG engine. Copyright (c) 2013, Hydrogen Energy Publications, LLC. Published by Elsevier Ltd. All rights reserved.