## 3 Results
### Carbon Dioxide (CO\({}_{2}\)) Emission  
Figure 16a,b show the CO\({}_{2}\) mass fractions and specific CO\({}_{2}\) emissions in all simulation cases, respectively. It can be seen from Figure 16a that, at each SA, the CO\({}_{2}\) mass fraction tended to increase as the IP increased from 0.01 to 0.02 m in both diesel and DF modes. Meanwhile, at each IP, the CO\({}_{2}\) mass fraction tended to decrease as the SA increased from 40 to 50 degrees in both modes.  
Figure 15: Foot emissions in all simulation cases: (**a**) soot mass fraction, (**b**) specific soot.  
As is widely known, carbon monoxide (CO) and CO\({}_{2}\) are carbon-based emissions, so their formation is directly dependent on the number of carbon (C) atoms contained in the fuel. Moreover, their formation is influenced by the combustion quality inside the engine cylinder. CO\({}_{2}\) is a product of the complete combustion of hydrocarbon fuels. In ICEs, hydrocarbon fuels are firstly oxidized by O\({}_{2}\) contained in the supplied air to form CO during the combustion process. CO then is oxidized to form CO\({}_{2}\) sequentially if there is still enough O\({}_{2}\) and the temperature in the engine cylinder is high enough. In other words, CO\({}_{2}\) formation is strongly influenced by the temperature and O\({}_{2}\) concentration in the engine cylinder.  
In this study, as can be seen in Figure 13, at each SA, the in-cylinder peak temperature tended to increase as the IP increased in both diesel and DF modes. The higher temperature inside the engine cylinder promoted the oxidation of CO to form CO\({}_{2}\). This resulted in increases in CO\({}_{2}\) formation as the IP increased in both diesel and DF modes, as shown in Figure 16a. At each IP, as can be seen in Figure 12, the in-cylinder temperature in the 50-degree SA case was the lowest. This resulted in the lowest CO\({}_{2}\) emission formed in this case, as shown in Figure 16a. In the other two cases (40- and 45-degree SA cases), although the peak temperature in both cases was almost the same, the temperature inside the engine cylinder during the expansion process in the 40-degree case was higher than that in the 45-degree case. The higher in-cylinder temperature in this working process of the engine promoted the formation of CO\({}_{2}\) from CO oxidation. This led to an increase in CO\({}_{2}\) formation in the 40-degree case compared with the 45-degree case, as can be observed in Figure 16a.  
When expressing the CO\({}_{2}\) emission in terms of specific values, the simulation result showed a different trend as shown in Figure 16b. It can be seen from the figure that the IP of 0.01 m with an SA of 45 degrees produced the lowest specific CO\({}_{2}\) emission in both diesel and DF modes. Using this setting of the injection parameter produced only 0.611 and 0.489 kg/kWh CO\({}_{2}\) emissions in the diesel and DF mode, respectively. This setting of the injection parameter is thus considered to be the optimal parameter that should be set for the fuel injector to reduce CO\({}_{2}\) emissions in both diesel and DF modes.  
Compared to the diesel mode, the DF mode helped to reduce CO\({}_{2}\) emissions due to the cleaner characteristics and fewer carbon atoms of NG fuel in comparison to the diesel oil. The reduction tendencies of CO\({}_{2}\) emissions when using NG as the primary fuel in DF modes compared to the diesel mode because of the many advantages of NG had been also reported in much previous research [10; 11; 12].  
Figure 16: CO\({}_{2}\) emissions in all simulation cases: (**a**) CO\({}_{2}\) mass fractions, (**b**) specific CO\({}_{2}\) emissions.