Optimization of Injection Parameters for a Free-Piston Engine Equipped with Flat-Head Combustion Chamber

The free-piston engine represents a paradigm shift in internal combustion engine technology, with its unique structure promising efficiency gains. However, injection parameters are one of the core elements of free-piston engine performance. This study employs computational fluid dynamics analysis to optimize the spray cone angle and start of injection timing for a two-stroke dual-piston opposed free-piston engine equipped with a flat-head combustion chamber. A three-dimensional transient model incorporating dynamic adaptive mesh refinement was constructed by using CONVERGE 3.0 software. The results indicate that a spray cone angle of 25° achieves optimal fuel distribution, yielding a peak indicated thermal efficiency of 42.14% and an indicated mean effective pressure of 9.08 bar. Crucially, advancing the ignition timing to 215°CA improves mixture homogeneity but simultaneously increases peak cylinder temperatures and NOx. Conversely, delayed start of injection timings reduces NO emissions by 58.6% at the expense of 8.2% indicated mean effective pressure. Comprehensive optimization shows that the combination of θ = 25° and SOI = 215°CA achieves 19.94 kW indicated power and 99% fuel evaporation, balancing performance and emissions. This study establishes a fundamental framework for optimizing injection parameters in combustion systems of free-piston engines, providing valuable reference for developing highly efficient, low-emission power systems in practical applications.