Effects of secondary hydrogen injection interval on combustion, heat transfer, and emissions in a diesel-pilot-ignited hydrogen free-piston engine

This study numerically investigates the effects of the secondary hydrogen injection interval on mixture formation, combustion, heat transfer, and emissions in a diesel-pilot-ignited hydrogen-fueled free-piston engine. The results indicate that the best mixture uniformity and turbulence intensity are achieved at 90° equivalent crank angle (SHCI-90ECA), with the dimensionless mixture fraction peaking at 9.3 × 10⁻³. This case presents the maximum cumulative heat release of 615 J near top dead center and a peak indicated thermal efficiency of 47%. Meanwhile, the minimum peak wall heat-transfer coefficient of 2896.59 W/(m²·K) is obtained under SHCI-90ECA, although this condition exhibits the largest absolute peak total wall heat-transfer rate. Although the highest NO mass fraction of 7.6 × 10⁻⁴ is observed due to elevated in-cylinder temperature and pressure, the final soot mass fraction remains extremely low at approximately 1.4 × 10⁻¹⁶. This work reveals the regulation mechanism of SHCI and provides a theoretical basis for high-performance and low-emission operation of hydrogen-fueled free-piston engines.