Study on nozzle hole number effect on airflow motion and mixture combustion in diesel ignition free-piston hydrogen engine

The diesel-piloted hydrogen combustion mode can ignite the hydrogen-air mixture through diesel compression ignition, effectively suppressing issues such as pre-ignition and backfire, and thus ensuring the stable operation of hydrogen-fueled internal combustion engines. In diesel-driven free piston hydrogen engines (FPHE), the number of hydrogen injection orifices plays a crucial regulatory role in fuel jet morphology, spatial distribution, and air-fuel mixing processes, thereby exerting a significant impact on the combustion efficiency, emission levels, and power output of the engine. To this end, this study combines experimental and numerical simulation methods to investigate the influence of four different injection orifice numbers on in-cylinder airflow movement and mixture combustion characteristics. The results show that with the increase in the number of injection orifices, the quantity and distribution range of in-cylinder vortices increase first and then decrease, among which the 4-orifice configuration exhibits the optimal performance in terms of airflow disturbance intensity and turbulent kinetic energy level. This configuration results in lower in-cylinder peak pressure and temperature; although it causes a slight combustion delay and slower heat release rate, the total heat release is higher. Analysis of performance and emission indicators reveals that both the 3-orifice and 4-orifice configurations achieve a combustion efficiency of over 98 %, with the 4-orifice configuration improving by approximately 0.61 % compared with the 3-orifice one. The 3-orifice configuration is characterized by a higher heat transfer coefficient (HTC) and faster heat flux growth rate, which exacerbates in-cylinder energy loss, leading to a lower indicated thermal efficiency (ITE) than that of the 4-orifice configuration—the latter increases the ITE by about 5.1 %. In terms of emissions, compared with the 3-orifice configuration, the 4-orifice configuration reduces nitrogen oxide (NO) emissions by 33.69 %, while the reduction rate of soot emissions is as high as 98.81 %. In summary, the 4-orifice injection configuration can optimize the in-cylinder airflow and combustion processes, achieve efficient and uniform combustion, and balance the requirements of engine performance improvement and pollutant emission reduction.