Reduction of carbon dioxide (CO₂) emission, which causes global warming, is an important guideline for vehicle engine development. There are two types of methods for reducing the CO₂ emission of a vehicle engine. The first involves improving engine efficiency. The second involves the use of a low-carbon fuel, i.e., fuel with high hydrogen to carbon ratio. Hydrogen-compressed natural gas blend (HCNG) has been researched as a low-carbon fuel. Given that thermal efficiency of an engine cycle increases with its compression ratio (CR), an HCNG engine with high compression ratio not only has high efficiency but also low CO₂ emission. However, unexpected combustion such as knock could occur owing to the increased CR.In this study, we investigated the knock and emission characteristics of an 11-L heavy-duty HCNG engine with a modified CR. A conventional CNG engine was fuelled with HCNG30 (CNG 70 vol% and hydrogen 30 vol%). The engine was operated at 1,260 rpm, under the wide open throttle condition, and with CRs of 10.5 and 11.5. The results showed that under a CR of 11.5, the engine's knock margin decreased because of higher in-cylinder pressure and temperature. Thermal efficiency improved with the increased CR, which caused a significant decrease in CO₂ emission. However, NOx emission increased slightly at a certain excess air-ratio condition.