## 3 Results and Discussion
### Engine Behavior under Oxy-Fuel Combustion  
Replacing N\({}_{2}\) with EGR (CO\({}_{2}\) and H\({}_{2}\)O) as the inert concentration of the oxidant in the engine combustion process implies a considerable change in the in-cylinder thermodynamic behavior, as shown in Figure 7. Due to the higher heat capacity at constant volume of EGR vs. N\({}_{2}\), a lower specific heat ratio (\(\gamma\)) is obtained in the intake charge gas. As a consequence, lower peaks of pressure and temperature are obtained at the TDC and at the peak of combustion in the oxy-fuel combustion case. Also, as a consequence of lower \(\gamma\), the indicated efficiency of the compression ignition cycle decreases; therefore, more fuel is required to produce the same torque and power for a given engine speed. Consequently, more oxygen and more EGR must be trapped inside the cylinder, as shown in Figure 7, i.e., the trapped mass for the same power is higher in the oxy-fuel combustion mode. It is worth noting that the effect of lower gamma of the EGR vs. The N\({}_{2}\) is so strong that even with higher trapped mass, the in-cylinder pressure peak in the oxy-fuel combustion is lower. Also, the effect of the lower gamma is evident in the gas temperature evolution during the expansion stroke (from TDC --angle equal to 0-- to IPO) due to the lower temperature decrease from the temperature peak.  
As a consequence of the changes produced in the chamber, the behavior of the intake and exhaust gases varies with respect to that obtained when the engine operates with air. The exhaust mass flow in the engine increases due to the higher in-cylinder trapped mass, generating more flow, which drives the turbines (TCH-1 AG and TCH-2 AG in Figure 2b), and therefore increasing the outlet pressure of the corresponding compressors, as reflected in Figure 8. As for the temperatures, the inlet temperature is the same in both the conventional and oxy-fuel combustion cases, since it is controlled by the coolers located after compressors to obtain a target value; on the other hand, the exhaust temperature is slightly lower in the oxy-fuel combustion case, a result of the combustion temperature reduction. The lower specific heat ratio in the oxy-fuel combustion case generates a lower expansion  
Figure 7: In–cylinder thermodynamic behavior of engine in oxy-fuel combustion compared to conventional combustion at full load engine operating point.  
effect as the piston travels from TDC to BDC. This, combined with a lower peak cylinder temperature (see Figure 7), results in an exhaust gas temperature similar to that of the conventional combustion case.  
As represented in Figure 9, where the main engine parameters have been represented, the above-mentioned phenomena impact the engine performance when running under the oxy-fuel combustion mode. The effect of the lower specific heat ratio (gamma) during the engine cycle results in less work produced per engine cycle and, therefore, in a lower indicated thermal efficiency for the same torque and power at each engine speed. Since the model control seeks to get the same reference model BMEP, the fuel demand increases to compensate for the lack of work produced, thereby increasing the Brake-Specific Fuel Consumption (BSFC).