## 4 Conclusions  
Various palm oil biodiesel blends (PD0, PD10, PD30, PD100) in a common rail direct injection diesel engine were used under low idle speed conditions (750 rpm) applying pilot injection. The combustion characteristics were analyzed and the regulated and unregulated gaseous emissions (VOCs and toxic aromatic VOCs-BTEX) were studied. Our conclusions are as follows:  
1. The nitrogen oxide (NOx) emissions of biodiesel blends were lower than that of pure diesel and NOx tended to decrease as the blending ratio increased. Soot opacity and hydrocarbon (HC) were reduced with an increasing blend ratio. Carbon monoxide (CO) varied with the engine load conditions. Under low load, CO emissions tended to decrease with increasing blending ratio and increased under high load.
2. The VOCs emitted from engine are mostly alkane and aromatic, and benzene and tetrahydrofuran have the highest emission ratios.
3. BTEX (i.e., toxic aromatic VOCs) were detected under all test conditions, and benzene has the highest emission ratio, followed by xylene, toluene, and ethylbenzene.
4. Benzene increased regardless of engine load under all blends except at engine load 40 Nm with PD100. At low engine load, benzene was increased while toluene and xylene were reduced. At high engine load, the levels of toluene, ethylbenzene, and xylene from test fuels blended with palm oil biodiesel were lower than those of diesel. And these increased as the increasing blending ratio. However, benzene from pure palm oil biodiesel under high engine loads were sharply reduced.  
When biodiesel was applied under low idle speed conditions, the oxygen content of biodiesel, which is an advantage, and high density and low LHV, which are disadvantages, affect the combustion and exhaust pollutants in a complex way. In particular, it was confirmed that different types of VOC emissions were detected. The emission of BTEX, toxic VOCs, under all fuel blending ratios has been confirmed. Benzene, which can cause cancer, took the highest portion of emitted VOCs and increased with the blending ratio of biodiesel. Toluene, Ethylbenzene, and Xylene were relatively low compared to benzene emissions and their emission tendencies varied depending on engine load conditions. Therefore, further research to reduce the levels of toxic VOCs, especially BTEX, from a diesel engine fueled with biodiesel should be carried out.  
[PERSON] suggested this research, performed the experiments, analyzed all experimental data, and wrote this paper. [PERSON] performed the data analysis and contributed to the discussion, and supervised the work and the manuscript. All authors participated in the evaluation of the data, and reading and approving the final manuscript. All authors have read and agreed to the published version of the manuscript.  
This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the government of Korea (MSIT) (No. 2019R1F1A1063154).  
The authors also thank the teachers in the Center for University-Wide Research Facilities (CURF) at Jeonbuk National University for their help in collecting some experimental data.  
The authors declare no conflict of interest.