## 3 Results
### Emission Characteristics  
#### 3.2.1 Pollutant Emissions  
As mentioned above, the diesel engine directly injects fuel into the combustion chamber at the end of the compression stroke, and an uneven air-fuel ratio is formed. Therefore, PM is formed in the rich region, and NOx is generated in the stoichiometric air-fuel ratio areas. It is generally known that the use of biodiesel improves combustion through the effects of oxygen, which increases NOx[18; 19; 20]. However, in the idle condition of this study, the deterioration of spray due to the high viscosity of biodiesel counteracts the reduced ignition delay due to the increase in cetane number and the combustion improvement effect due to oxygen content in the fuels itself. Figure 7 shows the result of NOx and PM. In the case of an engine load of 0 Nm and a pilot injection timing of 15 \({}^{\circ}\)CA, NOx was generated due to the rapid combustion of the pilot injection. In the case of an engine load of 40 Nm, the NOx was generated when the pilot injected fuel that did not burn during the separation between the pilot and main injection burned together with the main injection. PM decreased by reducing the local fuel excessive area and soot nucleus production due to the oxygen effect with an increase in the biodiesel blend ratio.  
The results of CO and PM are shown in Figure 8. The emission trend of CO differs depending on the engine load, while the emission pattern depends on the increase in the engine load. When the engine load is 0 Nm, the CO level decreased because the amount of intake air that can be mixed with the amount of fuel injected is sufficient, and there is suitable oxygen content in the fuel. However, as the blend ratio is increased (i.e., in the case of PD100) the deterioration of spray due to the higher viscosity of biodiesel makes more partially fuel-rich areas, so the level of CO increased. On the other hand, the CO level increased as the blend ratio increased for an engine load of 40 Nm. The injected fuel increased to an engine load of 40 Nm, but the amount of intake air is the same, so the fuel-rich  
Figure 6: COV of IMEP and IMEP by pilot timings: (**a**) pilot BTDC 20 \({}^{\circ}\)CA, main BTDC 2 \({}^{\circ}\)CA, (**b**) pilot BTDC 15 \({}^{\circ}\)CA, main BTDC 2 \({}^{\circ}\)CA.  
Figure 7: NOx and PM by engine loads: (**a**) engine load of 0 Nm and (**b**) engine load of 40 Nm.  
areas where CO is produced are not reduced by the effect of oxygen in biodiesel. However, when the blend ratio is much higher, the influence of oxygen increased, and the combustion pressure decreased. HC seems to be reduced due to the improved combustion caused by the presence of oxygen. Palm oil biodiesel contains oxygen in the fuel, which is more favorable than fuel and oxygen mixing conditions in the spray of pure diesel. At lower blend ratios, such as PD10 and PD20, HC levels of pilot injection at a timing of 15 \({}^{\circ}\)CA are lower than 20 \({}^{\circ}\)CA. However, at high blend ratios, such as PD100, the levels of 15 \({}^{\circ}\)CA are higher than at 20 \({}^{\circ}\)CA. The reasons for this are the pressure increase from combustion of the pilot injection and higher heat release rate of the main injection by poor spray development of the pilot injection at a pilot injection timing of 15 \({}^{\circ}\)CA. However, when the blend ratio increased, the combustion pressure and the heat release rate were similar to each other. The reduction of the HC is similar in these conditions, but HC is reduced because of the higher oxygen content in the fuel and the increased mixing duration for 5 \({}^{\circ}\)CA by the early pilot injection.