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Nowadays, automobile manufacturers are focusing on reducing exhaust-gas emissions because of their harmful effects on humans and the environment, such as global warming due to greenhouse gases. Direct injection combustion is a promising technology that can significantly improve fuel economy compared to conventional port fuel injection spark ignition engines. However, previous studies indicate that relatively high levels of nitrogen oxide (NOx) emission were produced with gasoline fuel in a spray-guided-type combustion system as a result of the stratified combustion characteristics. Because a lean-burn engine cannot employ a three-way catalyst, NOx emissions can be an obstacle to commercializing a lean-burn direct injection engine. Liquefied petroleum gas (LPG) fuel was proposed as an alternative for reducing NOx emission because it has a higher vapor pressure than gasoline and decreases the local rich mixture region as a result of an improved mixing process.

Natural gas produced from coal or biomass is known as synthetic natural gas (SNG), which is expected to replace compressed natural gas (CNG). In this study, we used an 11-l heavy-duty CNG engine in a feasibility study of SNG. SNG, which is composed of 90.95% methane, 6.05% propane, and 3% hydrogen, was produced for the experiment and used as fuel to estimate its effects on combustion and emission characteristics. The torque, fuel flow rate, efficiency, fuel consumption, combustion stability, combustion phase, and emissions characteristics obtained using SNG were compared to those obtained using CNG in an engine speed range of 1,000-2,100 rpm under full load conditions. In addition, an engine fueled with SNG was given an overall evaluation using the World Harmonized Stationary Cycle (WHSC) emission test. The engine's knock characteristic was analyzed at 1,260 rpm under a full load condition. The results showed that there was no difference in power output.