## 3 Results and Discussion  
### Combustion Characteristic of Recompression Internal EGR  
Figure 4 shows the temperature traces for three different recompression strategies: \"Base\" case, CIC V1, and CIC V2. In the recompression cases (CIC V1, CIC V2), the engine operates at a higher temperature than the \"Base\" case. The result is that there is a limited flow of fresh air and burned gas due to negative overlap. CIC V1 is less effective in enhancing the cylinder temperature than CIC V2, due to its relative larger intake valve lift and duration than in the exhaust valve. The maximum temperature in the case of CIC V2 is higher during the main combustion period.  
Figure 5 shows the intake flow variation for three different recompression strategies. In the case of CIC V2, there was a rapid back flow of burned gas through the intake valve. This comes from an increased flow rate through the intake valve above that of CIC V1. Additionally, the burned gas discharged to the intake valve, and the temperature of the in-cylinder gas dropped. At a crank angle of 470\({}^{\circ}\), hereafter fresh air is flowing into the cylinder and then the temperature of the trapped gas is increased. Therefore, it can be concluded that back flow in the valves is one of the main factors creating combustion performance.  
Figure 6 shows the total trapped mass traces for different recompression strategies: \"Base\" case, CIC V1, and CIC V2. Total trapped mass can be defined as the quantity of all chemical species (including air, fuel, EGR, residual gas, etc.) in the cylinder at the start of the cycle. In the recompression condition, the total trapped mass is reduced and the amount of the trapped mass in the case of CIC V2 is relatively more elevated before combustion, due to the increase in the intake valve lift and duration. In the case of CIC V2, when the intake valve is open at 350\({}^{\circ}\), the mass quantity drops rapidly, and the negative valve overlap creates the high temperature and causes burned gas to only flow back through the intake valve at TDC.  
As shown in Figure 7, the increase in in-cylinder pressure during the negative valve overlap indicates its oxidation by the recompression of trapped gas. However, the in-cylinder temperature and combustion pressure in CIC V1 and CIC V2 are lower than in the \"Base\" case, due to a decrease in  
Figure 5: Flow traces at intake valve for different recompression strategies.  
Figure 6: Total trapped mass traces for three different recompression strategies.  
fresh air at the next ignition. This result showed that the exchange of gas was more enhanced in the case of CIC V2 because of the decreased combustion pressure in the period and peak value than the CIC V1 case during the intake stroke.  
Figure 8 shows the pressure-volume (PV) cycle for three different recompression cases. The main feature of this diagram is that the amount of energy expended or received by the system as work can be estimated as the area under the curve in the figure. In the cases of CIC V1, its pumping loss has no area on the graph due to the very low volume efficiency during the intake stroke. It indicates a higher peak pressure than the case of CIC V2. As can be seen, increased inflow of fresh air to the cylinder of CIC V2 causes pumping loss, but the maximum pumping pressure is lower than in the case of CIC V1.  
Figure 9 shows that a decreased pressure and inflow of fresh air affects the increase in the ignition delay at the pilot injection of high-pressure fuel. Then, a decreased heat release rate and expanded diffusion combustion at main injection occur as changes in the cases of CIC V1 and V2, as characteristics of low temperature combustion.  
Figure 8: P-V diagram for three different recompression strategies.  
Figure 7: Pressure variation for three different recompression strategies.