## 1 Introduction
Many studies [29; 30; 31] show the possibility of reducing pollutant emissions, such as HC, CO, and PM. In addition to the effects on regulated emissions by biodiesel, many researchers [10; 32; 33; 34; 35; 36] are studying the possibility of reducing VOCs using biodiesel. [PERSON] et al. [10] used a common rail direct injection diesel engine by applying canola biodiesel under different engine loads and showed the possibility of reducing VOCs by biodiesel. [PERSON] et al. [32] reported the emission trend of VOCs and pollutants using waste cooking oil blends corresponding to 2%, 4%, 6%, and 8% by mass of oxygen content under the five engine loads at 1800 rpm. In that study, with an increase of biodiesel, benzene increased for each engine load, however toluene and xylene decreased. [PERSON] et al. [33] studied the effects of the 20% soybean oil biodiesel blends on VOC emissions, and reported a reduction in aromatic VOCs such as toluene and xylene. Also, a higher oxygen content in the biodiesel blend may enhance combustion efficiency, tending to lower VOCs, while oxygen blending increased the probability of oxygen VOCs. [PERSON] et al. [34] used a six-cylinder heavy-duty engine for studying aromatic hydrocarbons and reducing VOCs with biodiesel and its blends of 2%, 5%, and 20%. In that study, all BTEX levels reduced and the total reduced level of VOCs was 21.5%. The highest emitted aromatic VOC was toluene, followed by benzene, xylene, and ethylbenzene. [PERSON] et al. [35] compared the effect of the level of regulated pollutant emissions and VOCs on the Japanese-13 mode with waste cooking oil blends of 10%, 20%, 30%, and 100%. This research showed that aromatic VOCs were reduced with increasing engine load. Benzene increased with biodiesel blend ratio, even though toluene and xylene were reduced. However, many studies on biofuels are focused on medium and high speed and medium and high load conditions based on an analysis of the above literature. Some studies [37; 38; 39; 40] are performed at high idle, over 1000 rpm, with heavy-duty diesel engines using biodiesel. At present, research on low speed (especially idling) is still lacking. A lot of harmful emissions are emitted from engines under idling conditions due to the poor combustion environment. In particular, the poor atomization by biodiesel has a greater effect on combustion at low engine speed and low injection pressure. The condition of the lowest speed and the lowest injection pressure of the real vehicle engine is low idle. BTEX (Benzene,toluene, ethylbenzene, xylene), the toxic aromatic VOCs, emitted from engines of vehicles can directly affect people in the city at the low idle operation of real vehicles when parking or stopping at traffic lights. In an analysis of driving patterns in the city, the portion of idle operation is 17% [41]. In this condition, the injection quantity is a little higher than the optimum amount of fuel for stability. So, the oxygen content, the high viscosity, and other properties of the biodiesel affect combustion in a complex way. From the results obtained in actual vehicles and engines mentioned above, it was found that more VOCs were emitted at lower speed. The above studies were performed under relatively good conditions above medium speed and not in the low idle state of the actual vehicles. Therefore, to thoroughly investigate the combustion and emission characteristics of the diesel engine fueled with biodiesel blends under idling conditions, we applied palm oil biodiesel and its blends to a common rail direct injection diesel engine at the lowest speed of 750 rpm. The combustion and exhaust characteristics were analyzed, including the regulated and unregulated pollutant emissions (VOCs and toxic aromatic VOCs-BTEX).