## 4 Conclusions  
To the best of the authors' knowledge, this is the first detailed study available in the literature about oxy-fuel combustion in a 2S multi-cylinder cargo ship engine where, additionally, turbocharging, oxygen generation, and exhaust gases post-combustion conditioning sub-systems have been considered for the succeeding CO\({}_{2}\) capture process. The oxy-fuel combustion technology is a novel option for exhaust gas treatment in maritime transport, as it avoids NOx emissions and facilitates the separation of CO\({}_{2}\) from exhaust gases for subsequent capture. However, these improvements in emissions levels come at the expense of increased engine BSFC to obtain the original brake power and torque. This is because EGR, by replacing N\({}_{2}\) in the intake charge, has a lower specific heat ratio, which decreases the indicated efficiency per cycle and thus increases fuel consumption. In situ O\({}_{2}\) production in a marine engine is seen as a convenient solution for the implementation of oxy-fuel combustion onboard vessels due to the logistic hassle that supplying large amounts of O\({}_{2}\) from an external source would imply. The O\({}_{2}\) production system, based in MIEC membranes, involves two main subsystems that work together to condition the air for its separation process in the membrane: a heat exchanger network and a turbocharging system. Figure 17: Conclusive and complete schematic of the oxy-fuel marine engine showing the actual values from simulations at full load engine operating point (78 rpm). (**Top**) Temperature (\({}^{\circ}\)C). (**Medium**) Pressure (bar). (**Bottom**) Mass flow (kg/s). (**Inside and bold**) Power (MW). The heat exchanger system requires the implementation of combustion chambers in the exhaust gas and EGR flow lines to increase the heat transfer to the air, because the energy obtained from the flow lines coming from the engine is not sufficient due to the thermo-mechanical limits for exhaust gas temperature in 2S marine engines. The excess O\({}_{2}\) present in the exhaust gas stream is used to oxidase additional fuel with a two-fold benefit: increasing the temperature of the flue gases and reducing the concentration of O\({}_{2}\) for an effective posterior CO\({}_{2}\) separation. Although limited to the two lowest engine speeds, the combustion chamber in the EGR flow line is necessary, because the available heat in the exhaust gases is lower than at full load conditions. In relation to the turbocharging configuration selection, a 3-end design membrane was used for O\({}_{2}\) generation in the marine engine as it was initially assumed that the driving force to compensate for the missing sweep current in the MIEC would be a vacuum suction force on the permeate side. Nevertheless, during the evaluation of the various types of turbocharging configurations, it was concluded that the two-stage air compression system remains the best solution due to the high O\({}_{2}\) partial pressure achieved on the feed side, confirming the preference for it in previous related studies available in the literature. Along with the selection of the turbocharging arrangement, it is essential to optimize the performance of the configuration itself in order to find the most optimal turbocharger size ratio to obtain the lowest fuel consumption in burners. To this end, the present work proposes a methodology that consists of maintaining the size of the turbocharger that receives the air directly from the atmosphere equal to that of the air engine design, while sweeping the second turbocharger size with respect to this reference one. This methodology is followed because increasing the dimensions of the first turbocharger is not feasible due to its large size, and on the other hand, decreasing it implies having less air mass flow entering the complete system. Instead, downsizing the second turbocharger improves the air conditions for the MIEC membrane operation as long as it does not exceed the reduction limit beyond which the tendency is not favorable.