# Effect of PODE on Emission Characteristics of a China VI Heavy-Duty Diesel Engine
## 3 Results and Discussions
### Effects of EGR Rates on Emission Characteristics  
Figure 6 shows the thermal efficiency decreases slightly with an increase in the EGR rate. This is because as the EGR rate increases, the amount of exhaust gas in the cylinder increases, and the triatomic gases such as CO2 and H2O increase the specific heat capacity of the mixed gas, which means the temperature in the cylinder decreases. At the same time, due to the decrease in oxygen content, the combustion gradually deteriorates, resulting in a decrease in effective thermal efficiency. At the same EGR rate, as the PODE blending ratio increases, the effective fuel consumption rate also increases, which can be explained by the shortened combustion duration [7; 9]. Figure 5: PN emission of four fuels under different loads. Figure 7 shows the average in-cylinder pressure and instantaneous heat release rate curves of the four blended fuels at different EGR rates. The average in-cylinder pressures of the four blended fuels essentially show a downward trend as the EGR rate increases. The peak instantaneous heat release rate drops slightly as the crankshaft angle corresponding to the peak shifts back, as does the heat release start point. This is due to the fact that as the EGR rate rises, the amount of exhaust gas entering the cylinder rises while the oxygen concentration in the cylinder falls, slowing the combustion rate. During the compression process, however, the content of triatomic gases, such as CO2 and H2O, in the cylinder increases, increasing the mixture heat capacity and lowering the temperature in the cylinder. Figure 8 shows the CO emissions at various EGR rates. As the EGR rate increases from 0 to 20%, the CO emissions increase by 200%, 111.3%, 127.8% and 105.6%, respectively, corresponding to D100, DP10, DP20 and DP30. Because as the EGR rate grows, the intake oxygen concentration declines and the heat capacity of the in-cylinder mixture will increase, which leads to the peak in-cylinder temperature reducing and the CO emissions rapidly increasing [19]. A high oxygen content can facilitate the combustion and promote the oxidation of CO, so the emissions of CO when using DP10, BP20, and BP30 are higher than those of D100. Figure 9 shows that the HC emissions increase continuously as the EGR rate grows. The reason is that the peak combustion temperature declines and the combustion rate slows down due to the decrease in fresh charge amount and the increase in inert gas content in the cylinder, which results in an increase in wall quenching layer and HC emissions. Besides, unlike the results of CO emissions, blending PODE has no significant effect on THC emissions. Figure 6: Brake thermal efficiency (BTE) for four fuels at different EGR rates. Figure 7: Effect of EGR rate on in-cylinder pressures (P) and heat release rates (Q) of different blended fuels. (**a**) D100, (**b**) DP10, (**c**) DP20 and (**d**) DP30. Figure 10 shows NOx emissions with different EGR rates. From the chart, as the EGR rate grows, the emission of NOx significantly reduces. The reasons are as follows: the oxygen concentration of the in-cylinder mixture decreases and the higher heat capacity of the in-cylinder mixture causes the peak in-cylinder temperature to decline [24]. After blending with PODE, the NOx emissions further decrease. The PODE's influence on NOx emissions can be attributed to the competition between the decrease in the adiabatic combustion temperature and the increase in oxygen content. Due to the low blending ratio, the combustion characteristics of DP10 are similar to diesel. At EGR rates of 5%, 10%, and 15%, DP10 facilitates the combustion process and provides a favorable condition for NOx formation under low and medium load conditions so that the NOx emission of DP10 is equivalent to or slightly higher than that of D100. However, when EGR rates or PODE blending ratios increase, the reduction in combustion temperature (as shown in Figure 7) plays a dominating role in the formation of NOx, thus reducing NOx emission.