Experimental methodology for semi-closed cycle engines: Insights from an oxy-fuel combustion pilot plant for carbon capture

This work presents an experimental methodology for testing reciprocating internal combustion engines (RICE) operating under semi-closed cycle (SCC) oxy-fuel combustion (OFC), a technology enabling near-zero NO x emissions, strong reduction of CO and unburned hydrocarbons (uHC) in the raw exhaust gas stream, and inherent CO 2 capture capability. Unlike previous studies mainly limited to simulations or single-cylinder engines, the methodology is experimentally demonstrated on a full-scale 2.2 L automotive compression-ignition (CI) engine under representative operating conditions. The engine was adapted using high-pressure O 2 injection, controlled liquid water injection into the intake manifold, and an exhaust gas drying system to produce a high-concentration water-saturated and gaseous CO 2 stream. A dedicated measurement framework was implemented to quantify fuel and O 2 consumption, in-cylinder pressure, system pressures, temperatures, and gaseous emissions. Results confirm that stable SCC-OFC operation is achievable in a production CI engine with only minor hardware modifications, supporting retrofit applicability. Water injection increases the effective heat capacity of the working gases and moderates combustion temperatures; however, partial condensation and EGR stream substitution was observed, reducing the effective in-cylinder O 2 -to-fuel ratio and influencing combustion behavior. To address the measurement challenges of SCC with unknown gas composition, a dedicated methodology was developed to quantify gas flows and concentrations, supported by a Monte Carlo-based uncertainty analysis. Overall, this work establishes experimental guidelines for testing SCC-OFC in commercial engines and demonstrates capture-ready operation for direct CO 2 separation.