## 1 Introduction  
Fossil fuels are used in all areas of humanity and are primarily used as energy sources for internal combustion engines installed in automobiles, construction machinery, and ships. The consumption of fossil fuels has increased tremendously since their initial use. According to the research of US Energy Information Administration, 29% of the total energy is for transport purposes, and private transportation such as cars and trucks is highest [1, 2]. This increase in the use of internal combustion engines causes problems such as exhaustion of fossil fuels, air pollution, global warming, and rising oil prices. Oil production is now at the limit of production [3]. In addition, regulations on environmental pollutants emitted from internal combustion engines have increased [4, 5, 6]. Accordingly, developments of both mechanical engineering technology and new fuels for internal combustion engines have been conducted to address environmental pollution and the depletion of fossil fuels. Policies have been put forward that require renewable energy [7]. A representative example of renewable energy is biofuelsThere are various biofuels such as biodiesel, bioethanol, and GTL (gas-to-liquid) fuel, which can replace energy sources in all transportation sectors [8; 9; 10; 11; 12; 13; 14]. Biodiesel is produced by converting triglyceride, which is the main component of vegetable oil or animal oil, into an alkyl ester by reacting with alcohol [15]. Since vegetable oil has a high viscosity, it cannot be applied to the engine without pretreatment. When it is changed into the alkyl ester form, the physical properties become similar to petroleum diesel and the cetane number increases. Therefore, it can be used without mechanically modifying a diesel engine [9; 15]. To date, the biodiesel used worldwide has different main ingredients depending on the region. Examples of vegetable biodiesel include palm oil, jatropha, rapeseed, soybean, sunflower and coconut. Animal biodiesel and waste cooking oil are also used [9; 16; 17]. Many studies [18; 19; 20] have applied biodiesel blended with petroleum diesel to an internal combustion engine to study the performance and emissions of environmental pollutants. These studies show that applying biodiesel reduces the engine brake power and brake thermal efficiency (BTE) but increases brake specific fuel consumption (BSFC). In addition, hydrocarbon (HC), carbon monoxide (CO), carbon dioxide (CO\({}_{2}\)) and particulate matter (PM) decreased, but nitric oxide (NO\({}_{\rm x}\)) increases slightly due to the characteristics of biodiesel, which affects combustion pressure, heat release rate. Also, studies on volatile organic compounds (VOCs), which are unregulated environmental pollutants emitted from engines using biodiesel, are also underway [21; 22]. There is also active research on the morphology of soot from biodiesel and increasing interest in the fine dust generated [23; 24]. In this study, palm oil biodiesel was selected among many biodiesel. It is known that palm has the highest oil content among raw materials of biodiesel, so its production cost is low. In addition, palm has the highest production rate among raw materials, and the process of converting to biodiesel is the same as using other raw materials. It is known that the physical properties such as viscosity, cetane number, and heating value of palm oil biodiesel are better compared to other type of biodiesel. In addition, the results of test using palm oil biodiesel on the real vehicles recoded much cleaner exhaust emissions [25]. A study of the combustion characteristics and environmental pollutant emission characteristics using palm oil biodiesel was conducted by [PERSON] et al. [26]; the engine power decreased, and the amount of PM emission decreased as the blend ratio of palm oil biodiesel increased. However, the NO\({}_{\rm x}\) increased.