Study on the Influence of Diesel Fuel Substitution Ratio on the Characteristics of Dual-Fuel Free-Piston Engines

The diesel substitution ratio is a key parameter influencing the combustion characteristics and energy conversion efficiency of hydrogen diesel dual-fuel free-piston engines. This study develops a thermodynamic hydrodynamic coupled model for a dual-fuel free engine to investigate the effects of five substitution ratios (15%, 20%, 25%, 30%, and 35%) on in-cylinder mixture formation, combustion characteristics, and emission performance. The key novelty of this work lies in employing this fully coupled combustion-dynamics model to systematically optimize the hydrogen–diesel substitution ratio, which explicitly captures the critical feedback between combustion and the piston’s unique motion. The cumulative heat release served as the key quantitative metric. The analyzed parameters included the gas mixture fraction, turbulent kinetic energy, flow trajectories, in-cylinder pressure and temperature, combustion reaction rate, unburned equivalent ratio, cumulative heat release and its rate, heat release rate, and emission mass. The results demonstrate that the engine’s overall performance is optimal at a substitution ratio of 25%. At this ratio, a peak volumetric mixture fraction of 0.0088 was achieved with a broad distribution range, indicating significantly improved spatial fuel uniformity. The flow field exhibited organized swirl patterns that enhanced fuel dispersion. The peak in-cylinder pressure reached 7.2 MPa, which was 0.044 MPa higher than that of the 20% group. The combustion temperature remained stable, with a peak value of 1606 K, exceeding the 20% and 30% groups by 7 K and 16 K, respectively. The heat release phase was well-synchronized with the piston motion, ensuring a high proportion of premixed combustion for thorough fuel oxidation. Although nitrogen oxide (NOx) emissions were slightly higher, the reduction in soot was substantially greater than in the 20% group, leading to overall superior performance compared to the other substitution ratios. This study develops a thermodynamic hydrodynamic coupled model for a dual-fuel free-piston engine by leveraging the interaction between piston motion and combustion. This paper presents a novel strategy for optimizing the substitution ratio in a free piston engine via a fully coupled combustion-dynamics model.