## 3 Results and Discussion
### Effect of EGR on the Dual-Fuel Engine
#### 3.3.3 Emission Characteristics  
The effects of EGR rate on emissions are depicted in Figure 10.  
Figure 10: Effect of different EGR rates on emissions: (**a**) HC; (**b**) CO; (**c**) NOx; (**d**) Soot.  
Figure 9: Effect of EGR rate on the HRR.  
Figure 10a illustrates the effect of the EGR rate on HC emissions. It is evident that HC emissions increase as the EGR rate increases. This can be attributed to two factors. First, when EGR is applied, a portion of the intake air is replaced by recirculated gas, resulting in insufficient air for complete combustion. Secondly, an increase in the EGR rate leads to a reduction in the high-temperature area within the cylinder; both of these factors contribute an increase in to HC emissions. As discussed previously and depicted in Figure 10, there is minimal variation in HC emissions between a DMG substitution ratio of 0% and 15% at an EGR rate of 0%. Consequently, when the EGR rate is below 10%, the HC emissions of the original engine are similar to that of a 15% DMG substitution ratio, as demonstrated in Figure 10a.  
Figure 10b displays the relationship between the EGR rate and CO emissions. It is evident that CO emissions increase as the EGR rate increases. This can be attributed to two factors. First, an increase in EGR rate leads to a reduction in the in-cylinder temperature, which inhibits the oxidation reaction of CO. Secondly, the dilution effect of EGR decreases the oxygen concentration in the intake air, further inhibiting the oxidation of CO. However, the addition of DMG enhances the oxidation reaction of CO due to its higher oxygen content. Therefore, at a lower EGR rate (less than 7%), the CO emissions of the dual-fuel engine are lower than that of the original engine without EGR. This indicates that the presence of DMG helps to mitigate CO emissions even with the application of EGR.  
The variation in NOx emissions under the influence of EGR is displayed in Figure 10c. It is observed that NOx emissions drastically decline with an increase in EGR. For the dual-fuel engine with 20% EGR, NOx emissions declined 74.5% compared to that of the engine without EGR. This result strongly proves the efficiency of EGR on the inhibition of NOx generation. Additionally, at lower EGR rates (less than 4.9%), the NOx emissions of the dual-fuel engine are lower than those of the original engine without EGR.  
According to the information in Figure 10d, there is a noticeable increasing trend in soot emissions as the EGR rate increases. This can be attributed to the fact that the introduction of recirculated gas reduces the oxygen concentration, temperature, and pressure inside the engine cylinder, creating a favorable environment for generating soot. However, when comparing the dual-fuel engine to the original engine without EGR, it is found that lower soot emissions can be achieved by the dual-fuel engine, particularly when the EGR rate is less than 13%. This is because the high temperature and rich oxygen content of DMG significantly improve the combustion environment. This compensates for the impact of EGR on soot emissions, resulting in lower soot levels even with the application of EGR.  
In summary, the dual-fuel engine outperforms the original engine in power, fuel economy, as well as CO, NOx, and soot emissions, while it has similar HC emission levels as the original engine, once the DMG substitution ratio is 15% and the EGR rate is approximately between 5% and 7%.