Numerically supported process development for power density enhancement of hydrogen closed-cycle CI engines

Hydrogen closed-cycle engines combine hydrogen combustion with an argon–oxygen working-gas loop, offering a promising path toward high-efficiency, carbon-free, and nitrogen-oxide-free power generation. Previous demonstrations of this Argon Power Cycle (APC) concept have shown high indicated efficiencies, but low power density remained a central barrier to practical application. This paper presents a simulation-driven preliminary study on power density enhancement of a hydrogen closed-cycle CI engine. The study deliberately covers a very wide parameter range; its purpose is concept and process development, so the reported results should be read as trends and potentials. The long-term goal of the underlying project is a large-bore two-stroke closed-cycle engine; the single-cylinder research engine considered here serves as a model application to study the governing effects that must be understood for the design of the large engine. The investigation includes one-dimensional system simulation, four-stroke and two-stroke parameter studies, and three-dimensional CFD combustion modeling. Test-bench measurements provide the experimental basis for boundary and validation data. A 54-case parameter study shows that the volumetric power density ratio E v can be approximately doubled through hydrogen enrichment, compression-ratio adaptation, and elevated system pressure, constrained by mechanical peak-pressure limits; two-stroke conversion adds 60–80% further potential. The validation work identifies the caloric properties of the trapped working gas as a critical parameter: the species composition of the argon–oxygen loop directly determines the effective heat capacity ratio and therefore governs compression temperature, ignition delay, and pressure rise. The 1D system model quantifies the power-density potential and establishes thermodynamic boundary conditions for the subsequent 3D CFD analysis, which is applied to a selected operating point after reduction of the compression ratio to resolve the ignition process and heat release under well-defined boundary conditions. The CFD model, based on detailed gas-phase reactions and a combined H 2 /O 2 –diesel mechanism, demonstrates that combustion can be simulated successfully. An accompanying economic assessment indicates that efficient argon and oxygen recycling is an enabling condition for the concept. The results provide a validated methodological foundation for future scaling to larger-bore engines and further process development.