## 4 Conclusions  
In this study, the effect of internal EGR by variable valve actuation with post injection using a full-circuit GT-POWER model verified by experimental data was investigated to predict auto-ignited combustion performance that includes in-cylinder pressure and temperature, heat release, ISFC, IMEP, and NO\({}_{\rm x}\) emission. Conclusions obtained by this study can be summarized as follows:  
1. [leftmargin=*]
2. Recompression internal EGR leads to negative valve overlap effects and a higher temperature in engine operation, due to its trapping of burnt gas and limited air flow, creating a high temperature inside the cylinder. The earlier the exhaust valve closes during the exhaust stroke, as in the case of a larger NVO, the greater the amount of the hot exhaust gas from the previous engine cycle would be trapped in the cylinder. This leaves less room in the cylinder for the in-coming fresh air mass, thereby increasing the cylinder charge temperature while decreasing the oxygen level in the cylinder charge. As a result, there is an optimal setting of NVO for each operating condition to achieve the best engine performance.
3. The CIC V2 case is used for enhancing the internal EGR rate, which is reduced in both the lifting and duration of the exhaust valve without control of the intake valve, thus the operating temperature is decreased overall, but a higher temperature is seen at the main combustion period than in the \"Base\" and CIC V1 cases. This is caused by the inflow of more fresh air while keeping  
Figure 19: Comparison on NO\({}_{\rm x}\) reduction by applying different variable valve control strategies.  
internal gas in the cylinder and increasing the discharging of burnt gas through the intake valve. Therefore, effective work has risen due to a pressure drop in the intake stroke and the rising of pressure at the main combustion, compared with the CIC V1 case.
3. For the use of thermal energy wasted when adjusting recompression by variable valve actuation, post injection with three cases was applied with total injected fuel of 20% based on CIC V2. Each post injection shows a similar volume efficiency, but retarded post injection timing caused an increase in IMEP, as well as a reduction of both ISFC and NO\({}_{\mathrm{x}}\). In the case of post injection at 320\({}^{\circ}\), when increasing the pressure and temperature for both intake and exhaust valves, which were closed in the recompression period, the highest engine operating temperature was seen, but there were lower emissions of NO\({}_{\mathrm{x}}\) than in other post injection cases.
4. The recompression internal EGR is effective in utilizing heat energy by enhancing the EGR rate by about 10%. Additionally, recompression with retarded post injection is beneficial to IMEP with NO\({}_{\mathrm{x}}\) reduction, even if CIC V1 with no-full lift and duration of the intake and exhaust valve has an internal EGR rate of about 2% higher than that of CIC V2 with no-full lift and duration of the exhaust valve.
5. Finally, the internal EGR scheme with variable exhaust valve actuation in a diesel engine is advantageous to apply post injection from the viewpoint of utilizing thermal energy. However, since the ISFC increases according to post-injection conditions, it is necessary to consider the maximum use of exhaust heat energy for NO\({}_{\mathrm{x}}\) reduction.  
This research was supported by the R&D project on Industrial Core Technology (2017) of MOTIE (Ministry of Trade, Industry and Energy) in Republic of Korea.  
[PERSON], [PERSON] and [PERSON] conceived and designed the numerical analysis; [PERSON] and [PERSON] produced analytic model and performed the simulation; [PERSON] and [PERSON] analyzed the data and wrote the paper; [PERSON] supervised and advised all parts in this paper.  
The authors declare no conflicts of interest.