To counteract the harmful effects of vehicle emissions on humans and the environment, such as global warming due to greenhouse gases, there is a focus on gaseous fuels as an alternative energy source of transportation. Heavy-duty natural gas vehicles are widely used to improve the air quality of urban areas in Korea because natural gas has the advantage of low greenhouse gas emission levels. However, more in-depth study is required in order for clean fuel vehicles to hold a dominant position over well-developed diesel vehicles. It is difficult to meet reinforced emission standards with only a lean combustion strategy without an aftertreatment system in a lean-burn natural gas engine.Hydrogen-natural gas (HCNG) blends have been proposed as an alternative to improve fuel economy and emissions of lean-burn natural gas engines, since they have a wider flammability range and faster burning speed. HCNG blends could also play a role as a technical bridge for the hydrogen era. In the present study, the feasibility of HCNG engines was assessed with the use of compression ratio and valve overlap changes as part of an effort to develop an HCNG engine with an 11 L heavy-duty lean-burn natural gas engine. Torque and emission levels were evaluated, in light of the requirement for commercialization, at the maximum torque operation point. Torque and NOx emission levels meeting the required specifications and emission standards were achieved with optimization of operation parameters.