## 3 Results and Discussion
### Effect of Post Fuel Injection in Recompression Internal EGR  
By adjusting the internal EGR applied by variable exhaust valve actuation, a great volume of heat energy occurred and was generally wasted, but it can be used to achieve lower emissions, activation of the after-treatment system, and stable operation of low temperature combustion. This study investigated the possibility of VVA that involved post fuel injection for improving thermal efficiency in the three cases. The points of post injection started (1) at 70\({}^{\circ}\) containing a maximum quantity according to the total trapped mass; (2) at 200\({}^{\circ}\), with both the lowest pressure and temperature and the ending of the back flow through the exhaust valve during the recompression process; and (3) at 320\({}^{\circ}\), with the raising of pressure and temperature by closed intake and the exhaust valve during the recompressing process. Furthermore, the quantity of post fuel injection was set to 20% of the total fuel mass in both the pilot and main injection periods. As a reference, the specific cutoff point for which the second injection is small enough relative to the main injection to constitute a post injection, rather than a generic split injection, is not well defined, but a maximum of approximately 20% of the total fuel in the post injection is consistent with most existing self-described post-injection studies [13]. Figure 10 is the result of the temperature traces for different recompression strategies with post injection. Post injection at 70\({}^{\circ}\) in the exhaust stroke caused an increase in temperature. In the case of post injection at 320\({}^{\circ}\), it was decremented that a high temperature occurred by potential heat, which is the decreasing of temperature during NVO, which then affects gas exchange efficiently. Additionally, post injection at 320\({}^{\circ}\) was performed under conditions that included a lower burning mass than in other cases, so it showed the highest effect by potential heat. Figure 11 shows the result of the total trapped mass traces for different recompression strategies with post injection. In the case of post injection at 320\({}^{\circ}\), it shows a higher variation in trapped mass, due to its internal gas flow activity with a high temperature.  
Figure 10: Temperature variation for different recompression strategies with three post injections.  
Figure 9: Heat Release Rate traces for three different recompression strategies.  
Figure 12 shows the result of the heat release rate traces for different recompression strategies with post injection. It shows the highest heat release at the pilot and main injection points in the case of post injection at 320\({}^{\circ}\). This is caused by increasing the burnt fuel with post injection by decreasing the total trapped mass. In addition, it can be concluded that post injection is one of the main factors influencing thermal efficiency increased by a diffusion combustion area.  
Figure 13 shows the result of in-cylinder pressure variation for different recompression strategies with post injection. In the case of post injection at 320\({}^{\circ}\), it shows the lowest pressure that results from reducing the pressure rate at the point of main combustion by increasing the gas exchange.  
Figure 11: Total trapped mass traces for different recompression strategies with three post injections.  
Figure 12: Heat release rate traces for different recompression strategies with three post injections.  
Figure 13: Pressure variation for different recompression strategies with three post injections.