## 3 Results and Discussion
### Emissions Characteristics  
#### 3.2.1 Regulated Gaseous Emissions  
Table 7 summarizes the emission characteristics of NOx, PM, HC, and CO, which are regulated gaseous emissions. NOx emissions of palm oil biodiesel blends under all engine load conditions were lower than those of pure diesel fuel. Under pure diesel fuel condition of 0 Nm engine load, NOx produced 258 ppm, PD10 decreased by about 1.2% to 255 ppm, PD30 decreased by about 5.0% to 245 ppm, and PD100 decreased by 2.7% to 251 ppm. And under 40 Nm engine load, NOx of pure diesel fuel was 835 ppm, PD10 decreased by about 1.2% to 825 ppm, PD30 decreased by about 5.9% to 786 ppm, and PD100 decreased by 2.8% to 812 ppm. In particular, with PD100, the slow combustion of pilot injection and the ignition delay of the main injection increased because of poor atomization caused by the high viscosity of palm oil biodiesel so that the premixed combustion increased, resulting in increased generation of NOx. Opinions and results on the generation of NOx in diesel engines with Biodiesel are divided [29; 59]. [PERSON] et al. [59] reviewed the NOx emissions of diesel engines fueled with various biodiesel. In that study, it was said that the NOx generation of biodiesel blends was complex and not conclusive and there were many factors that can influence NOx emissions such as fuel cetane number, density, volatility, degree of unsaturation, the chemically bound oxygen content, equivalent ratio or aromatic fuel composition. [PERSON] et al. [60] used four biodiesel (palm, jatropha, soybean, beef tallow) and its blends (0%, 5%, 10%, 20%, 50%, and 100% by volume) to compare the emission characteristics of NOx after application to diesel engines. In this study, NOx decreased with increasing the biodiesel blend rate. The reasons were explained as follows: (i) Reduced atomization and less premixed burn because of the high viscosity and a reduced volatility, (ii) lower cylinder temperatures for the higher blend percentage, (iii) an increase in unsaturated hydrocarbons, reducing energy release rate during oxidation through strong bonds, and (iv) prompt NOx reduction as oxygen present in fuel oxidizes radical combustion. [PERSON] et al. [61] tested with mahua oil ethyl ester and reported a 12% reduction compared to pure diesel fuel. It also reported that the generation of NOx is sensitive to oxygen content, adiabatic frame property, and spray characters. [PERSON] et al. [62] also applied various biodiesel fuels to a single-cylinder 4-stroke diesel engine, and reported the emission of NOx reduced when biodiesel applied. Using biodiesel or its blends is generally known to decrease PM, HC, and CO due to the effects of the oxygen content of biodiesel [29; 30; 31]. Smoke opacity decreased from 3.1% (PD0) to 1.2% (PD100) at an engine load 0 Nm and from 4.8% (PD0) to 1.8% (PD100) at 40 Nm. HC also decreased by about 26% from 54 ppm (PD0) to 40 ppm (PD100) at an engine load of 0 Nm, and by about 60% from 62 ppm (PD0) to 25 ppm (PD010). Most of the studies on the application of biodiesel found reductions in smoke opacity and HC. The emission of CO depended on the engine loads. At an engine load 0 Nm, CO was reduced in the fuels with a low blend ratio but increased with the pure palm oil biodiesel. The CO emission of PD0 was 493 ppm, PD10 was 475 ppm, and PD30 was 439 ppm, and PD100 was 492 ppm. Conversely, at 40 Nm, CO of the fuels with a low blending ratio tended to increase but reduced with the pure palm oil biodiesel. The CO emission of PD0 was 241 ppm, PD10 was 270 ppm, PD30 was 315 ppm, and PD100 was 245 ppm. The following analyses can be made on the causes affecting the properties of CO emissions by the emission results and combustion characteristics. At a low engine load of 0 Nm, the increasing blend ratio of in biodiesel can make close to complete combustion by the added oxygen from biodiesel. Thus, the CO emissions of PD10 and PD30 tend to decrease compared to PD0.