## 3 Results and Discussion
### Engine Results
The unburned hydrocarbon emissions showed a similar trend, which can be noticed because the new combustion configuration produces lower HC (in mg/s per cylinder) than the initial diesel reference case, which is in agreement with the combustion efficiency observed in the cumulative HR curves presented in Figure 14. In the preliminary stage, the baseline oxy-fuel case generated extremely high levels of unburned hydrocarbons, due to strong jet-wall interaction that slowed down the combustion combined with the formation of very rich zones near the squish, with little  
Figure 14: Comparison between in-cylinder pressure, RoHR, and normalized cumulative HR for the reference diesel case, baseline, and optimized oxy-fuel combustion case. O\({}_{2}\) availability, that stopped the combustion process. However, the configuration found by the optimization procedure was able to promote the mixture formation and the O\({}_{2}\) consumption as is corroborated in the last part of this section. Additionally, the net indicated efficiency was also evaluated. It is net indicated efficiency, because for this analysis only the high pressure indicated work is taken into account, because the study is focused on the closed cycle and the combustion process. It can be seen that the changes in the system produced an increase in the net indicated efficiency of the engine compared to the baseline oxy-fuel combustion (from 26.6% to 35.7%). However, this value is still far from the initial value of the conventional diesel case (40.1%). Because the carbon dioxide molecule has a larger specific heat capacity compared with oxygen and nitrogen, the increment of CO\({}_{2}\) in the combustion chamber due to the higher EGR rate used in the oxy-fuel combustion concept, will vary the specific heat capacity of the mixture. Therefore, the adiabatic expansion coefficient (\(\gamma\)) when operating with EGR + O\({}_{2}\) is lower than considering pure N\({}_{2}\); then the efficiency in the oxy-fuel case is lower compared to the reference one, although they have similar cumulative HR. For all the positive efforts made during the study, this is one weakness of the concept that still remains. On the other hand, it must be highlighted that this is compensated by the absence of NOx emissions in the exhaust gases. Moreover, because there is no N\({}_{2}\), the CO\({}_{2}\) can be easily separated from the water vapor (by water condensation) and compressed until it reaches a supercritical condition. This means capturing it as a dense supercritical liquid of high purity (more than 95%). Therefore, the increment in BSFC does not mean an increment in CO\({}_{2}\) emissions. Figure 15 shows the energy balance for each of the three cases presented in Table 9. For the baseline oxy-fuel combustion case, it can be seen that some amount of the energy that is lost due to the unburned fuel, because it cannot find the oxygen available in the chamber. On the other hand, the optimized oxy-fuel combustion case is able to burn most of the fuel recovering part of the energy that was wasted in the Baseline oxy-fuel combustion case (and comparable to the one from the reference diesel-air case). However, higher losses were found through the walls, where the most significant difference is in the liner region due to the difference in compression ratio and bowl shape. Also, an important part of the energy is lost in the exhaust gases as a consequence of the lower adiabatic expansion coefficient of the gas. Lastly, in order to understand the distribution of the mixture in the system, Figure 16 shows contours of the oxygen concentration inside the combustion chamber during the combustion process. The columns of the figure correspond to the instants of 10% (CA10), 25% (CA25), 50% (CA50), 75% (CA75), and 90% (CA90) of burned mass, and the rows represent each one of the three simulated cases. For the reference diesel case, the oxygen concentration available at IVC is lower than the other cases due to the air composition (oxygen and nitrogen).