## 3 Results and Discussion
### Emissions Characteristics
#### 3.2.3 Toxic Aromatic VOCs, BTEX  
BTEX was detected under all test conditions in this study. Xylene has three isomers, i.e., meta-xylene, para-xylene, and ortho-xylene, depending on the location of methylene (CH\({}_{3}\)) in the benzene ring. Xylene was analyzed by combining the emission ratio of all these xylene isomers in this study. Figure 6 and Table 10 summarize the emission ratio of BTEX based on engine load and test fuel conditions. Total BTEX accounts for approximately 50% or more of the total VOCs emitted under each test condition. The largest proportion of each test fuel discharged was benzene, followed by xylene, and toluene. The smallest percentage emitted was ethylbenzene. In the studies of [PERSON] et al. [65] and [PERSON] et al. [66] (conducted using waste cooking oil biodiesel) and [PERSON] et al. [35] (conducted under Japanese-13 test mode), benzene was also the largest emission followed by xylene and toluene. Unlike the above studies, [PERSON] et al. [34] used a heavy-duty diesel engine to show that toluene was emitted most, followed by benzene, xylene, and ethylbenzene. The above results indicate that the emissions of BTEX vary depending on the fuel used, the engine used, and the test conditions. In this study, benzene tends to increase with an increasing rate of the blend of palm oil biodiesel. The emission ratio of benzene increased from 26.7% for PD0 to 36.5% for PD100 at an engine load of 0 Nm. The benzene emission ratio also increased from 38% for PD0 to 43% for PD30 at 40 Nm. [PERSON] et al. [67] showed that biodiesel produced higher benzene emissions than when pure diesel fuel was used. They reported benzene emissions of 6.8 g/kWh for B20, which is a 62% increase compared with 4.2 g/kWh for diesel. A study by [PERSON] et al. [35] with waste cooking oil biodiesel also showed that benzene increased with increasing blend ratio of biodiesel under any engine load. Many studies were indicated the major factor of increasing benzene was the low exhaust temperature. [PERSON] et al. [68] pointed out that benzene emissions increase under low exhaust temperature conditions. [PERSON] et al. [65] and [PERSON] et al. [35] pointed out that the addition of biodiesel at lower engine loads lowered exhaust temperatures, thereby increasing benzene emissions by slowing benzene's oxidation while increasing the oxygen content due to the biodiesel. In other words, the exhaust temperature and oxygen content of biodiesel have a combined effect on benzene emission. However, as shown in Table 4, the emission ratio of benzene increased in this study, even though the exhaust gas temperature was the same (390 K) for all blends at an engine load 0 Nm. It also increased when the temperatures increased from 485 K to 493 K using PD0 to PD30 at an engine load of 40 Nm. The exhaust gas temperature was reduced to 489 K at PD100 with an engine load of 40 Nm, but the benzene emission ratio was reduced. This can be inferred from the fact that the increased oxygen content or the high exhaust gas temperature is not the main factor for reducing benzene under idle conditions with low injection pressure and low engine running speed.