Experimental analysis of effects of load, intake pressure and equivalence ratio on a Semi-Closed Cycle Oxy-Fuel Combustion engine for carbon capture purposes

In this paper, an experimental evaluation was conducted to determine the effect of engine load, intake pressure, and the O2-to-fuel ratio (λ) on a Semi-Closed Cycle Oxy-Fuel Combustion (SCC-OFC) reciprocating internal combustion engine (RICE). To this end, tests were carried out at low and mid-high loads in a commercial compression ignition (CI) RICE retrofitted to OFC, sweeping intake pressure and λ at a constant temperature while employing water injection for evaporative cooling. On the one hand, it has been demonstrated that the CI engine can operate across a very wide range of EGR rates—up to 95% at low loads—although with excessive O2 dilution, as inherently high cycle pressure means high EGR and high trapped mass. On the other hand, at mid-high loads, suitable O2 concentrations are achieved even at high cycle pressures, thereby enabling efficient combustion. The cycle operating intake pressure is identified as a key variable in the optimization and control of closed-cycle RICEs. Furthermore, it was shown that the exhaust dry CO2 concentration can be isolated from variations in load, water injection, or EGR, maintaining a constant level through the control of λinj. A critical correlation was observed between the quantity of liquid water ingested by the engine and the deterioration of combustion efficiency (an increase in CO and uHC emissions), presenting a fundamental challenge for closed cycles operating with high wet EGR rates. The extended ignition delay caused by OFC CI - RICE offers a promising avenue for the adoption of highly reactive fuels, such as high-cetane biodiesel, paving the way for the integration of biofuels with source CO2 capture directly from the highly concentrated flue gas of SCC-OFC RICEs.