## 4 Conclusions  
In this study, a numerical methodology for the optimization of an engine using oxy-fuel combustion was presented. This work aimed to find a proper combustion system architecture to improve the efficiency and reduce the pollutant emissions in a CI engine for oxy-fuel combustion concept application. The methodology, based on the CFD-NS coupling optimization algorithm, considered the combustion chamber geometry, injector system specifications, and air-management system to design a specific combustion system. The main conclusions obtained from this investigation can be summarized as follows. * The CFD-NS coupling optimization method has demonstrated to be an adequate procedure for redefining the baseline combustion system of a CI engine when it is operated under oxy-fuel combustion concept. This method includes several routines to automatize the optimization process. * The new combustion system changes the baseline geometry to a bathtub shape that improves the engine efficiency and reduces the heat transfer losses. The number of injector nozzles changes from 10 to 7, allowing an improved interaction between the fuel and the oxygen inside the combustion chamber. The injection pressure is higher than the baseline, which leads a better fuel atomization and evaporation. The spray angle is higher and is better adjusted with the piston bowl geometry. With this new configuration, the swirl number should be lower in order to avoid the interaction between the sprays. Lastly, the SOI must be adjusted to control the maximum in-cylinder pressure limit. * The optimized combustion system for oxy-fuel combustion improves the efficiency levels obtained by the baseline configuration (from 26.6% to 35.7%) while reducing soot emissions from 11.37 mg/s to a virtual zero. Soot avoidance is very important for this engine concept because exhaust gas energy must be recovered in proper regenerators (heat exchangers) to increase air temperature and separate O\({}_{2}\) from N\({}_{2}\) in  
Figure 17: Temperature distribution inside the combustion chamber for a several combustion periods (CA) comparison between the reference diesel and optimized oxy-fuel combustion case. the MIEC. Therefore, soot must be minimized in the combustion chamber to prevent heat exchangers from clogging. * Similarly, the new combustion system is able to reduce CO emissions up to similar levels of conventional diesel combustion. In addition, C\({}_{x}\)H\({}_{y}\) are below the conventional diesel combustion with air because there is no tradeoff between hydrocarbon oxidation and NO\({}_{x}\) reduction. This is a big advantage of oxy-fuel combustion because it allows for the minimization of pollutant emissions from the origin in the combustion chamber, because all design objective points are in the same direction of maximizing hydrocarbon oxidation. Having a virtual zero harmful emissions engine outside the combustion chamber makes the objective of obtaining a very high purity CO\({}_{2}\) easier. If some oxidation catalyst were needed for exhaust gases, after-treatment would be easier to design as well, due to the high temperatures, and lack of NO\({}_{x}\) and oxygen excess. However, the efficiency levels are significantly lower (35.7% against 40.1%), compromising the fuel consumption. Nevertheless, the possibility of CO\({}_{2}\) capture by simply condensing the water vapor allows for the decoupling of the BSFC increment from CO\({}_{2}\) emissions. Future works will be in the direction of minimizing BSFC while keeping the virtual zero emissions achievement. In view of these trends, oxy-fuel combustion on a CI-based engine architecture can be considered an interesting solution for reducing the environmental footprint of transportation. This combustion concept allows us to completely avoid NO\({}_{x}\) emission by removing nitrogen form the oxidizer, while achieving very low pollutant emissions (soot and CO) if the combustion system is accordingly optimized.